The Post Office were not insensitive to the growing number of alternative telephone designs that were emerging. In 1965, it adopted the Trimphone for premium residential use. It was designed by STC, who called it the Deltaphone. A novelty was a tone ringer in place of a bell, which increased in volume the longer the phone was rung. The hook-switch doubled as a carrying handle. The Trimphone was not capable of supporting plan set working and its uniform tone ringer sound made it unsuitable for open plan offices. But its most famous drawback was its light weight, which required one to hold the telephone base when dialling – something that wasn’t easy to do when the handset could not be cradled on the shoulder as earlier designs could be. This was rather an ‘own-goal’ by the Post Office, since it insisted on using its traditional dial mechanism rather than the low torque Italian dial that STC had intended should be used. Another problem was that the handset was hollow with the microphone close to the earpiece, so if one wanted to speak without the other party hearing, one had to cover both ends of the handset. A triumph of design over utility!

Another novel feature was that the dial was illuminated by a luminescent tube of radioactive tritium gas. With most surviving Trimphones, this gas has long since leaked away, but the radioactivity meant that restrictions were placed on the mass storage of Trimphones in PO stores. When these dials were finally disposed of, this issue became a problem for BT. The Health & Safety Executive were not happy with skip loads of these dials at the back of BT stores!

The Trimphone took its name from these features Tone Ringing Illuminated Model.

In 1978, a keypad version was produced, but a slightly deeper body shell was needed to accommodate the dialling electronics, but at least it didn’t need holding down.

Eventually, in 1979, an MF version was produced. By using the more modern MF generator circuit without bulky inductors, it was possible to squeeze everything in, but even so, a third body shape, intermediate between the dial and SC keypad version was used.

Body comparison

Body comparison

You can’t keep a good design down. In 1980, STC relaunched the Trimphone as the Deltaphone, both dial and keypad versions being leather-clad. They were also supplied by the Post Office as part of the Special Range Telephones. Later in the 1980s, the basic design as relaunched as one of the Phoenixphone range. New bodies and colours were produced on refurbished innards. These designs were styled “The Snowdon Collection”. The black and grey model shown above was one of six colour schemes.