A crash course in using GPO telephones in the modern world

As tempting as it can be to grab one of those shiny replicas we often see in nice-things-shops I can tell you it is well worth having a real one. If could just spare a little time for a few boring details.

I recently purchased purchased a couple old GPO 746F rotary dial telephones (a.k.a. NZPO 100 for New Zealanders like me). A true icon of British design, but suspiciously archetypal of the earlier American Model 500 telephone.

There is only one reason to have these around the house: Because they look pretty. Being an of an engineering mindset, pretty things have to be functional too. Even if I barely use them, in the event that I do take one off the hook – it’s got to work perfectly.

To get these phones working perfectly in the modern age – there are a few modifications which need to be made to get the best experience. These are:

Replace Carbon Granule Microphone with 21A Electret Microphone

A couple of 21/A electret microphones.

These phones originally sold with prehistoric carbon granule microphones which worked well… for the first few years or so. BT Soon realised they were a little flaky so issued an official upgrade – part number ’21A’.

These were made by a few manufacturers, each differing in appearance. All work good. For some reason the red ones are particularly sort after, I believe this is because they fit into the earlier 300 series phones, so not worth paying the premium for if that’s not what you’re working on.

Pulse to DTMF (tone dialing) Conversion

Most phones sold on eBay are advertised as ‘Converted’. This just means that the phone has been rewired for use with the plug and socket system. In every other way these phones remain prehistoric.

I wouldn’t waste time with a standard ‘converted’ phone, because (assuming you have a BT landline, because it won’t work on anything else) – you’ll be able to be dial numbers, but as soon as you’re in a menu i.e. internet banking or a customer service line, you’re stuffed as these systems cannot understand pulse dialing.

My build of the Boris Cherkasskiy / Arnie Weber ‘DIY’ pulse-to-tone converter

To get around this we must add an extra device to the inside of the phone – a pulse-to-tone (DTMF) converter. The most common product which does this is a Rotatone which is available from a number of vendors.

For the serious DIYer – take a look at this.

Pulse to tone converters also add speed-dial / redial functionality too, which is a bonus because while dialing a phone number on a rotary dial is a novelty at first, it quickly gets tedious.

Bell on/off switch

This is optional but the bells in these phones (as lovely as they sound) are particularly loud. You’re going to want a way to mute them in the event you don’t want to take that pesky sales call.

This is all a bit technical. Can’t I just pay someone to do all of this?

Sure. This company will do it all for a price – you’ll get a nice phone but it will likely contain some reproduction parts. Select ‘Bell On/Off switch’ and ‘Pulse to tone’ option when purchasing, and you won’t have to lift a finger.

BT won’t be providing analogue lines by 2025

There is a solution – using an FXS to SIP Analogue Telephone Adapter. There are heaps of products available (I’d recommend a Cisco SPA 112).

The back of a Cisco SPA112

It creates a fake phone line, allowing your phone to work over the internet. You’d then have to subscribe to an online SIP provider (your ISP may even provide this service) to get your phone number.

In my own setup, I have a Cisco 2900 series, with a VIC3-4FXS which’ll do any cadence and ring frequency you like. The other bonus  with this approach is that (assuming correctly configured) the phones can be assigned internal extension numbers and call each other directly. Note: UC License is required to use one of these modules.

For serious techies – A Cisco VIC3-4FXS provides the ultimate IP solution for vintage phones.

Line and ringing voltage on these cards is lower than a BT exchange would produce, however this does not seem to result in any noticeable difference.

One last think you’ll need – a few of these:

An RJ-11 to BT adapter – because ATA’s don’t have BT sockets on them. These typically (should) contain the ‘ring capacitor’ normally found in BT master sockets. Without it, the bells in the phone wouldn’t work.

10 thoughts on “A crash course in using GPO telephones in the modern world

  1. Hi Matt,
    I’m not quite clear why BT’s switch to IP lines will make DTMF phones unusable (typically a converter is installed at the residence). However, re. your concern that “one potential snag you may encounter using one of these devices is that it’ll likely ring your phone with American cadence:”

    I have the SPA3102 (the SPA112 is listed by Cisco as the successor), and can program ringtones. I have a US ringtone for one line, and a Dutch for the other (I had it ring “Australian” for a while). I can also adjust the impedance for a Dutch or US telephone. Assuming the SPA112 is similar, there is a tremendous amount of flexibility programming the SPA.


    1. I have a crappy no-brand TA which doesn’t have configurable cadence. Agree it’s unlikely you’ll find one that isn’t configurable.

      I guess it’s possible TA’s may be provided by ISPs 5 years from now, but I would have through that’d be unlikely. It’s not even common now!

  2. Easiest way is to use a Grandstream Handy Tone/HT series Analogue Terminal Adapter (ATA). The 5XX and 7XX both accept pulse dialling telephones and can be set up with UK tones and cadences including the old ‘purring’ dial tone for those that remember it back in the 1950’s and 1960’s before it was changed wit the introduction of Trunk Dialling. Even the currently produced Handy Tone/HT 8XX series works with pulse dialling and I’ve managed to get them to add the ‘reverse’ NZ dialling as an option with their latest firmware. We’ve been using the Grandstream ATAs on our replica of the old UK public network (CNet) where we still use the old dialling codes ( London is still 01-XXX XXXX !) and the old GPO Speaking Clock of 1960’s is still on 8081 on most ‘main’ exchanges or TIM(846) in the big cities. Network stretches as far as New Zealand where their old codes are still used! No ‘line rental’ or ‘call charges’ !! Even British Telecom have lines off the system – several 0533 XXXXX numbers rather than their present day 0117 XXX XXXX numbers! I administer the network for the World (except +1 land – the North American Numbering Plan Area). If you want more info – email me. Network has been going for 15 years and getting bigger – goes into over 25 countries!

    1. Hi ,I have just purchased a 1967 gpo phone which is working fine apart from the volume on the earpiece is a little quiet is there anyway of improving this .
      Regards Alistair

      1. If you also have a grandstream terminal adapter, as Ian has, in the post you replied to… Then under the web UI of the FXS port tab, look for “Gain” pull down menus for TX and RX…. Using these you are able to adjust the volume of your receiver/earpiece and transmitter/microphone.

        1967 is the year that the GPO 746 phone was introduced…. so I suspect that is the phone that you have…. if that is the case… then it has an automatic regulator that was designed to accomodate being close to a telephone exchange… and receiving strong signals…. and far away from the exchange and reciving weak signals…. No user adjustment is required for this regulator. The earlier phones, the GPO 706…. made from 1959 had a regulator that could be installed in two orientations. One to reduce the sensitivity. If you look at the wikipedia entry for GPO telephones… and scroll down to Type 706, you’ll see an explaination. if your phone is a 706… then maybe the regulator has been installed in the orientation to cut down you signals.

  3. Hello I’ve only just heard about the changeover and that traditional phones won’t work after 2025. I’m not at all technical but love retro so my home has 4 vintage phones 2 USA 2 Brit wired in. Do these converters that are Being mentioned plug in between the phone and its socket Or do they have to go straight to router? ( which is behind tv) would I have to re wire internet cables to each phone ?

    1. If it is a simple pulse / decadic to DTMF converter they go between the phone and the wall aocket on analouge phone lines.
      If you need to use the phone on a VOIP system the phone plugs into an ATA ( analouge telephone adapter) which has an ethernet connection for your network.

  4. We have a couple of 700 series phone working perfectly through a Siemens Gigaset 1000TAE, which is a DECT to wired adaptor. Pulse dialling no problem and the ring is fine. The only snag is that the 1000TAE has been out of production for years and they are a little hard to find, though German eBay can normally find one … at a price.

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