In 1972, what became known as System 3 was launched in London,and in South Lancs in 1973
This system offered a higher degree of privacy to the previous two systems as now one subscriber could usually no longer hear the call of another.
Most details gleaned from POEEJ July 1972 and PYE 1975 Type LW15FM radiophone sales sheet
Each radio capable of tuning to 10 (of the 37) channels (1 calling for the whole of greater London,3 for traffic at each of the 4 Transmitter sites
Selective calling by means of a 5 digit code, signalled with 11 tones between 1060Hz and 2600Hz tones, on the calling channel transmitted from 3 of the 4 transmitter sites one after the other. The Central transmitter site ( located on an 11 storey telephone exchange 1.6km south of the thames near waterloo), does not transmit the calling channel.(164Mhz)
Radiophones alert the operator that they want to make a call with a short burst of 2400 Hz tone on a free traffic channel
At the end of a call a similar 2400 Hz tone is sent
When the Radiophone user finds a free channel after in response to a call on the calling channel, a short burst (150ms-400ms) of 2600 Hz tone is sent on a free traffic channel.
Good Pictures at http://www.qsl.net/gm8aob/pages_2/lw15fm.htm