Telecommunications signals are carried on a cable (111) which carries mains electricity to a consumers' premises. A signal coupler (SC) and a subscribers' interface unit (SIU) are provided between the mains cable (111) and conventional subscribers' terminal equipment (25A, 25B, 25C) adapted for full duplex communication with a local telephone exchange. The subscribers' interface unit SIU converts telecommunications signals between this duplex form (typically analogue at audio frequencies) and time division duplex (TDD) form at a carrier frequency in the range 1 MHz to 20 MHz for coupling to the mains cable (111). In the example, TDD interface means (40) provides cordless telephony digital standard formatting/deformatting and modulating/demodulating at a carrier in the band 864 MHz to 868 MHz, and a frequency converter (50) converts between this band and, typically, 2 MHz to 6 MHz.