Abstract: A telephone communications interface adapted to provide a data link for receiving and transmitting serial data via FSK modulated carrier signals over telephone lines to a computer. An operational amplifier in a duplexer circuit has a variable impedance feedback circuit, which is automatically adjusted by a computer subroutine to adjust the amplifier gain, so as to compensate for variations in the telephone company line impedance. The gain is adjusted to minimize the amplitude of any transmitted carrier signal, which might appear at the input of the conventional band pass filter for the receiver carrier signal.