In the past, the General Post Office had a near monopoly (apart from one town) of the British telephone system. Subscribers to the service had to rent a handset from the GPO.
Most of these telephones used a dial to enter the number (some people still say "dial" with reference to entering telephone numbers to this day). The telephone most people will think of when they think about diaphones will be the GPO 706 and later versions. We have two GPO 746s (or rather 8746s as we will explain below) in our retro office equipment collection.
The 746 was introduced in 1967 as an improvement on the 706L. The main external difference was an improvement in how the telephone could be carried, as it had an integral cradel for the receiver. This meant you could more easily carry the telephone around with you when you wanted to look cool like a 1970s detective. It also improved how the handset rested on top of the telephone.
The 746 also had a built-in regulator which could not be removed as it could with the 706L. This regulator was used to vary the current flowing through the receiver and transmitter. This could avoid problems for subscribers who were too close to a telephone exchange - too strong a current could make things too loud - and could also reduce interference from strong radio signals. The regulator was an add-on component to earlier telephones but built into the main circuit board of the 746.
The 746 was available in a range of colours, we have red and brown examples. We did used to have black and grey ones too but they seem to have gone to the telephone graveyard in the sky. The 746 remained available for a long time, later examples could be modified to use the New Plan plug and socket (which is still used by BT - who took over from the GPO - and other telephone companies to this day). These telephones were called 8746, both of our telephones are so modified.
BT lost their monopoly over handsets in the 1980s. Subscribers were free to buy their own handsets and plug them in. The 8746 could still be used though, indeed i continued to use mine well into the 2000s.
