In 1981, the Post Office became British Telecom and lost the monopoly on extension telephones. In 1985 it lost the monopoly on the prime instrument too. In 1986, BT lost the monopoly on the supply and maintenance of extension wiring.
A key requirement of liberalising phones was to move away from hard-wired phones to a plug and socket system. True, plug and socket phones had been in use for many years (eg on Plan 4 and on the Special Range Telephones). However a cheaper system was needed allowing machine crimping onto the cord. Quite why the US plug and socket system could not have been used has been the subject of much debate. The UK adopted a 6 pin system, though generally only 4 are used on most plugs.
The simple phone became a 3 wire instrument – the ringing being fed via a capacitor in the master socket. UK telephones thus moved from having low impedance bells suitable for series connection, to high impedance bells or ringers capable of parallel working up to a Ringer Equivalence Number of 4 (though more is usually possible on shorter lines). The reason why the third ringer wire was used relates to the fact that, in 1981, all phones used loop dialling and by feeding the ringer wire to each phone ensured that the parallel phones did not degrade the loop pulsing of the phone in use. It also allowed any phone in the house to suppress ‘bell-tinkle’ in all the others, a feature less important today.