Abstract: A system for phase division multiplex duplex communication over a two-wire circuit has a master terminal and a slave terminal. The master terminal comprises a modulator and a demodulator jointly connected to a first end of the two-wire circuit and arranged to transmit an outgoing AM-signal with an essentially non-suppressed carrier component constituting a pilot signal and to receive an incoming AM-signal with a considerably suppressed carrier component having a frequency identical to the frequency of the pilot signal. An oscillator circuit is arranged to supply the modulator and the demodulator with a first and secnd, respectively, oscillator signal having a respective frequency identical to the frequency of the pilot signal and having such a relative phasing that at the demodulator the phase of the second oscillator signal is phase displaced 90.degree. relative to the phase of the pilot signal.