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 who designed and had manufactured a series of telephones.
The General Post Office released it's 706 model telephone in 1959, a modernised version of earlier GPO telephones with inspiration taken from contemporary American designs. The 706, and it's later update the 746, remained in production for decades and can still be used today if retro-fitted with a New Plan BT socket.
The 706 was fitted with an optional regulator which could reduce the sensitivity of the telephone if the line was too close to a telephone exchange or even if the telephone was picking up interference from nearby strong radio transmissions. The 746 had this regulator built into it's circuitry.
One variant of the 706 was the 706L which had a lettered dial. This was useful in the age of letter coded telephone exchanges. I have black and ivory examples of the 706 dialphone in my collection.
