Tech Flashback: Landline Sounds & Telephone Teardowns

I’ve always been interesting in telephones and networks, but I’ve never really thought of becoming a telephone collector. Rather unexpectedly, through a process of not-throwing-things-out (it’s not hoarding, I swear!), I now have a few phones but not much to do with most of them.

Then I realised the main reason why – I’ve not had access to a real landline for the better part of a decade. After ditching dial-up for broadband, we went straight to “naked” ADSL2+ which meant no line-rental cost, but also no actual landline. We were able to compensate for this with SIP VoIP through an ATA, but even that got very little usage. As a result, I realised there are many sounds and experiences we’ve since lost with the transition to more modern forms of communication.

After setting up my home PBX, it would make sense to run a few demos just for the sake of preserving the sound and feel of a landline, seeing as these are going to become practically extinct with the full cutover to NBN happening within the next year or two.

The Landline Experience

In the not so distant past, a telephone call was the most convenient way to interactively get in touch with someone who may be a long way away. At one point in time, almost everybody had a landline – only the most privileged may have a mobile number, whereas the lesser-privileged may have a pager number which you can call to leave a number or a message. The cost of having this land-line was the cost of “line rental”, which at this time is hovering somewhere about AU$30 per month with all calls billed additional to this. Owing to the economics, there’s no reason to have a landline when mobile telephone service can be had “all calls and SMS inclusive” and with 1GiB of data for AU$10 per month. The introduction of the NBN also means that copper land-lines are being deactivated as well.

To use a landline telephone, it’s a case of:

  • lifting the handset
  • listening for the dial-tone – if it’s there, the line is working and ready to accept digits
  • dialing the number – on older rotary pulse-dial phones, this means putting your finger into the corresponding number hole, rotating it to the finger rest and letting the dial return to its rest position, while it “clicks” the number onto the line at a rate of 10 pulses per second. On newer touch-tone (DTMF) phones, this means pushing the buttons that correspond to the telephone number in sequence.
  • listening for the ring-back tone – which lets you know that the dialled number is now ringing. Alternatively, you could get a busy indication to let you know that the line dialled is engaged so you can try again later.
  • hearing the phone get answered – then you can have your chat.
  • hanging up the phone – perhaps you’ve finished your conversation or the other end has replaced their phone “on hook” and you’re just listening to the tone indicating this, so you hang up your phone, clearing the line for an incoming call.

To receive a call:

  • the telephone will ring – depending on the phone, you might have a “real” bell, or an electronic “warbler” or selectable tones. This alerts you to an incoming call.
  • after the first ring, you can receive caller ID on suitably equipped phones – this only works if you’ve paid for the caller ID service and you have a phone with a caller ID decoder inbuilt. This sometimes doesn’t work if there’s too much line noise.
  • you can choose to answer the call by lifting the handset – then you can have your chat.
  • or you can ignore the call, which will ring … ring … ring … ring … ring …
  • hang up the phone once you’re done – this way you can receive another call.

None of this is too radically different to what you might have on a modern mobile telephone, but there are a number of key differences:

  • You don’t have a dial-tone anymore. You don’t hear the “hang-up” tone either.
  • You aren’t restricted to an old fashioned bell or limited electronic ringer.
  • Caller ID is taken for granted and just works “immediately” rather than being a paid option and only being received after the first ring.
  • There’s no mystery with caller ID either – if you didn’t pay for the service or didn’t have a decoder, you would answer every call just in case!
  • Voice quality is better on modern devices thanks to HD Voice between supported networks. On a traditional telephone, analog crackles, hiss, inducted noise from AM broadcast stations, etc would be sometimes audible along with level differences on longer lines.
  • You don’t have the same feeling of picking up the phone, punching in the number (or dialling the number on a rotary dial), or slamming down the phone on someone you didn’t want to talk to.
  • Voicemail is free – with landline phones, if you wanted to catch “missed” calls, you’d need to invest in an answering machine. Or if not, you’d need to pay extra for a service which records the last-missed-call number and allows you to retrieve it. In Australia, I still remember the TV campaign for this service, *10#.
  • There’s no such thing as a text message! But on the upside, landlines carry faxes, TTY and modem signals relatively well.

For those who may not have placed an international call before, it’s also of note that each country often has their own variations on tones to indicate call progress. As a result, Australian landlines have a particular “sound”, inspired from the British network, that you don’t get by default from some “emulated” phone lines from ATAs. If you’re interested, the database of tones from the ITU can be downloaded here.

Even though I don’t have a landline, I’ve configured my ATA to emulate one to a high level of fidelity, so here’s a call recorded from my modified cordless phone to my answering machine that shows the Australian dial-tones and ring tones.

Call to Answering Machine

Caller ID on Landlines

There are a number of ways to implement caller ID on landlines which include low-speed modem data or DTMF tones in-between rings. For compatibility reasons, the first ring is sent as standard, but in-between the first and second ring, the caller ID data is sent. Australia seems to be using a “Bellcore” FSK based data at 300bps. As I’ve never had the chance to examine it – I decided to hook up my ATA’s FXS port to my Rohde & Schwarz RTM3004 to see what it looks like.

The Australian ringing signal is 25Hz sinusoidal (as far as I know), with a cadence of 0.4s on, 0.2s off, 0.4s on, 2s off.

In the case of the ATA, the line idles at close to -48V (as expected), with the ringing amplitude of about 85V above and below ground. The caller ID data is in there … but you can’t see it because of the wide voltage swings involved.

There’s a good hint of it if we zoom in on the DC-coupled recording, but it’s not likely we’d see the detail so lets switch over to AC-coupling and try again – letting the regular ring voltage clip.

The caller ID data burst can be seen in the middle of the rings – sloping upward due to the AC-coupling frequency response.

The signal starts with a 01010101… preamble sequence, the FSK is clear.

… towards the middle, we can see real-data being passed. I didn’t bother decoding it, but it doesn’t have a particularly big amplitude – about 1.6v peak-to-peak. It’s definitely enough to satisfy my curiosity.

“Generic” Extension Cord and Bell

The thing that really got me excited about telephony again was rediscovering this old “generic” extension lead and bell. I remember my parents using this when I was young – it’s a very “analog” piece of technology.


It’s quite a simple device – just one on-off slide switch on the side that allows you to use the unit just as an extension with no bell feature. The plug has approval number #RA87/146.

There is a modular jack at the other end for plugging in the phone, with only two pins available suggesting that the unit only carries the tip-ring pair. The rear had space for a label which has long been lost, as well as a provision for a pair of wall-mounting screws.

As such connectors are getting rare, I could connect the unit backwards through the modular plug as long as I don’t touch the pins on the other side. Or I could use one of these very rare adapters (Approval Number RA85/102) which converts from the 600 series to modular plug.

The inside is rather uncomplicated, as expected. The main “star” of the show is a bell from Chang Cheng.

The underside shows a rather interesting logo and the fact it is Made in Taiwan (Republic of China).

The PCB has a lot of holes but only very few components – two diodes, a resistor, a switch and a capacitor.

The capacitor is rated 1uF with a 250V rating.

It seems like the PCB allows for a number of other configurations, possibly to meet the needs of other countries’ telephone networks.

Inside the bell is a solenoid coil with a pin – it vibrates back and forth, striking the bell at two points from what I can tell.

Behold, the lovely sound of the analog bell – the way a phone’s supposed to ring.

Telstra TouchFone TF200

The Telstra TouchFone series are somewhat iconic, as a “rental” telephone from Telecom Australia well throughout the 90s. You’d see these around everywhere and the ring is somewhat recognisable as well. The phone features some speed-dial memories, which apparently uses some current from the phone line leading to the phone occasionally (potentially) disrupting dial-up connections. This particular unit was “ex-business” and was found discarded in a still-working but dirty condition. It’s got such a severe sun-tan that it probably needs some retr0bright … not that I can be bothered to do it.

The phone doesn’t have many controls around the side – just a ring-tone volume control. There is a vent at the top, to allow the ringing noise to escape the casing.

This unit is branded Telecom Australia, manufactured by Exicom, not a name that I immediately recognise. One of the rubber feet has been lost. On the underside, there is a three-position switch for ringer pitch, but only two seem to work. There is also an impedance switch and a tone (100ms/600ms) and decadic (pulse) dialling switch.

The phone itself is pretty light and opening it up reveals an AWA 87603 and AWA 87830 marking on the plastic moulds. That’s quite an interesting find along with the two speakers – one for the ringer, the other for the keypad “pip” which acknowledges every button press.

To save money, it seems they’ve extended the membrane keypad to add an additional button for the hook switch. This relies on a pivoting piece of plastic with a levered rounded protrusion that presses against the flexible plastic contact.

From the only thing I can see with a date code, I suspect this unit is Week 25 of 1991, making it 27 years old. It still works, although slightly crackly. Surprisingly it seems that the hybrid is already a “silicon” type, not being implemented in a wound transformer as you might expect of older telephones.

The unit rings like a “warble” with both low setting and high setting.

“Generic” Cheap Handset

A very late model, just about the cheapest and most basic a telephone could get. The line cord reaches all the way into the handset which contains all the circuitry necessary to operate the telephone.

The keypad nestles inside the arched area of the handset, with the hook switch being a protrusion near the microphone which is held “closed” by the weight of the phone on a desk or in a wall cradle that has been long lost. Because this unit is “all in one”, I suppose this is a unit that you would especially avoid during a thunderstorm to prevent any chance of being “shocked” through the phone.

Because of its screw-less plastic construction, I didn’t open it up as I would have likely caused damage to it. However, I did make a recording of its weak, shrill-sounding tone.

Leader 724 PABX Desk Phone

This particular phone is intended for PABX use and was abandoned for disposal at a technology clean-up. Severely yellowed, it still works, although like the TF200, it seems to be a little crackly and susceptible to RF interference from Wi-Fi access points and DECT cordless telephones alike. The unit features speed-dial memories, a neon ringing indicator and mute capabilities.

The ringer can be set between off, low and high positions, with a “data port” on the side allowing for connection of dial-up modems to the phone without unplugging the phone.

The main line input is on the top side, with the handset cord coming out of the other.

A look at the rear shows a date code – probably Week 3 of 2004. The unit has a tone/pulse and line length selection on the rear with a ringer pitch potentiometer that felt quite stiff. There is a battery compartment, but a label over the top claimed that no battery is necessary – indeed there’s no contacts behind the cover. Despite saying it is for PABX, it operates just fine off the FXS port of the ATA – so it’s possible that it’s not impedance optimised for regular lines.

The construction inside seems to suggests that the design is rather adaptable, modular and somewhat kludged to meet whatever requirement is necessary. The PCB seems to accommodate a 9V battery due to the cut-out even though it’s not used. Other wires are “tacked” on for additional features, such as the neon indicator.

A cable tie also seems to secure a line filter for the input, but it’s not enough to stop interference from Wi-Fi access points.

The unit uses all-through-hole components on a paper type PCB, again, based on a silicon hybrid. This one has a Holtek HT9320H dialler chip. Surprisingly, there is a soldered-down Renata lithium coin cell (CR2430) that’s still reading about 3V even today. This is almost certainly used to hold the speed-dial memory. The hook switch seems to be a repurposed “power” type switch with the click-retention mechanism removed.

The front keypad is secured with so many screws, I couldn’t be bothered to disassemble it, so I’ve left it as it is. I don’t really want to break anything …

In case you’re wondering what’s inside the handset – not much. A speaker, an electret microphone and a bit of metal for weight and balance.

This is how the Leader 724 sounds like when it’s ringing.

Uniden XSA660 Cordless Phone

I tore down the XSA660 in an earlier post, but seeing as I mentioned radio scanning in a recent post, I thought I’d make some comment about early cordless phones and provide some sample recordings from an SDR.

Older cordless phones such as this operated in analog NFM in the 30-40Mhz band. While it was illegal to listen into telecommunications, I’d guess that a lot of scanner owners probably did listen in as it was an easy target for gossip. The joke was that your scanner neighbour would gift you a cordless phone as a way to learn all about you and perhaps even steal your identity. Such basic phones featured no protection – not even voice scrambling – against such evavesdropping which can happen even accidentally due to channel reuse. There was about 10 channels for use – the above shows me hopping between channels by hitting the scan button.

The only thing digital about these phones was the line control feature, including some sort of special rolling coding, so you couldn’t pick up your neighbour’s line and make calls on their line.

When listening on the base channel, it would sound like this – including the data-train paging the phone during ringing and the audio from the handset coming back out from the base for side-tine. Listening to the higher handset-to-base channel would sound like this, featuring only the handset audio and control signals. Likewise, this is what hitting the channel scan button during a call would sound like.

As a basic phone, you only had the choice of two ring tones – this one or this other one.

Uniden WDECT3115 Cordless Phone

The next cordless phone I owned was one I purchased from a liquidator at a low price. It’s a pretty sophisticated unit with speakerphone ability and a large selection of ring tones. Unfortunately, it’s a WDECT unit, meaning it operates in 2.4Ghz and interferes with Wi-Fi. If it were not for this fact, I’d be more fond of it. It seemed to be a fairly popular model with a space-age design.

The handset is much more compact than the analog type, with a much shorter antenna due to the higher operating frequency. The frequency hopping and digital nature also makes eavesdropping extremely improbable. This particular unit has smaller buttons, a nice LCD screen for Caller ID and provision for a belt clip attachment and external earpiece.

The base has a very curvy shape, along with wall-mounting options. The underside accepts the power and telephone cables, but with a way to route the cables securely and neatly. It’s quite a good design, with charging indicator and find-button on the side.

The handset uses a three-AAA Ni-MH rechargeable cell for power. The base has a REN of 0.1, which is not unexpected due to the electronic nature of the telephone.

To my surprise, the unit is based on a DSP Group chipset (DUL36107UAB11AQC) – the same company making chipsets for modern VoIP ATAs.  The PCB is dated Week 52 of 2007, whereas the chipset is dated Week 48 of 2007.

The keypad is surprisingly well-done – having a separate plastic sheet with carbon contacts which interface with the silicone pad – this way, liquid ingress is less likely and replacement of the carbon contacts is a possibility.

Looking into the base, the unit has two antennas in the curved section which provides for some reception diversity, improving coverage. There is a corresponding DSP Group chipset.

The base actually has two boards – the rear board is the line interface, including some protection against surges and a tubular “reed” relay for switching the line.

There are a few test points on the back – I suspect CID+/- may be test points for caller ID signal.

The rear of the main board shows that there is an RF canned module with castellations, soldered to the mainboard. This suggests that the RF module may be pre-approved or shared between other units.

Compared to all other phones, this one is relatively advanced, featuring quite a few melodies – twenty to be exact:

While it doesn’t ring like a proper phone, it does ring in its own “retro” way, kind of like old polyphonic mobile phones used to.

Uniden DECT1015 Cordless Phone

I’ve also torn down the Uniden DECT1015 in the past, which is an even more modern but basic cordless phone. Unfortunately, there’s only two ring tones – Flicker and Clatter which sound somewhat crappy out of the beeper.


I never intended to collect telephones, but faced with a pile of phone gear that hadn’t seen much action, I felt a little nostalgic about the sounds of the landline network and telephones in general. Now I’ve got some decent recordings of the “real” ring of a telephone bell, to the electronic rings from the later units for a rather “retro” feel.

I guess you could load it onto a smartphone, but you still won’t have the same satisfaction of punching in a number or slamming down the phone …

About lui_gough

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4 Responses to Tech Flashback: Landline Sounds & Telephone Teardowns

  1. Krzyś says:

    Text messages over PSTN are possible, actually.

    Not sure if in Australia, though.

    It is defined in ETSI ES 201 912 (either protocol 1 or 2).

    Asterisk supports it as well:

    I’ve got a DECT GAP phone with SMS capability. I’ve seen also other specimens of SMS-capable phones (all advanced DECT GAP cordless models, though). I’ve never used it though, as I believed that my telco (Orange Poland) demands additional monthly fee for the service. It seems I was wrong – sending does not involve any monthly fee; receiving is bound to Caller-ID in a sense, that is if you have Caller-ID enabled, then you’ll receive SMS as text; otherwise you’d receive it as voice call in which some Text-To-Speech engine will read it to you (Caller-ID is needed, so the phone could pick up the call – some phones require one or two digit less than SMS Center number to answer the call)

    I’ve just tried sending some SMSes, to no avail. (I had proper SMS center entered, so it was either me, or the person that I bought the phone from, who tried playing with it earlier). I suspect that it does not work either because the service is discontinued (I can’t find anything that confirm this suspicion, but OTOH I can’t find any confirmation that it is still available), or because the phone I’m using is British. In one try I’ve picked up handset of telephone connected in parallel and after dialing SMS Center number there was just a silence on both ends. Maybe I should sniff whether it dialed correct phone number and record the call with some voice modem.

    • lui_gough says:

      In Australia, it’s never been marketed for home use as far as I know. I’ve never seen SMS receive over fixed line at all here. I don’t think any carrier implements it for general use, but it’s not inconceivable that you could create a similar gateway using a modem of sorts or make your own protocol :).

      However, Telstra payphones are apparently Siemens X1/X2 units which do offer SMS over the PSTN and seem to do so via some modem-based protocol, possibly as you’ve described or possibly through something proprietary to that particular payphone model. They also use the same modem to transmit service information to a monitoring centre. However, payphones (for obvious reasons) don’t seem to accept SMS.

      – Gough

  2. sparcie says:

    Ah this brings back memories, especially the bell, as we had an old school rotary phone (with real bell) when I was a kid.

    I’ve actually found out by accident that you can send a SMS from a mobile to a land line, the person on the receiving end said a computer read it out to them!

    • lui_gough says:

      That’s a very interesting find in regards to SMS-ing a landline. I wonder if that’s a carrier-specific service (e.g. provided by Telstra), as “Visual” Voicemail was one of those carrier bonuses where some of them will transcribe voicemail messages into text for more convenient reviewing via their own app or sending via SMS instead. Maybe I could try it, but somehow I doubt it would work for me … also I wonder if it will try again if you missed the call … *scratches head*.

      – Gough

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