From BBC tech show “tomorrows world”. The device is basically a cordless extension to a PABX (private phone system) using a modified VHF or UHF radio. It would then have been analogue and vulnerable to interception with radio scanners. Unfortunately it didn’t take off because the govt didn’t have any spare frequency allocations (the blue light services had whole chunks of VHF / UHF and what was left over was still NATO allocations due to the Cold War).
The research is being carried out at Marconi (then a promiment manufacture of civillian and defence comms equipment) under strict monitoring by Home Office/DTELS. The old style tape recorder is being used for evidence the transmissions were authorised, it is a Nagra 4 portable which will still set you back €600 today for a second hand model.
