A subscriber line interface circuit includes a switched hybrid circuit (1) operable during a POTS mode when the associated subscriber is using a POTS station and in an ISDN mode when the subscriber is using an ISDN station. The hybrid circuit (1) includes a first amplifier circuit (22) for coupling a signal at the two-wire port thereof to its four-wire transmit port, and a second amplifier circuit (27) for coupling a signal at the four-wire receive port of the hybrid to the two-wire port. A high termination impedance is synthesized at the two-wire port during the POTS mode and a low termination impedance at the two-wire port during the ISDN mode. The signal from the four-wire transmit port is converted to a digital signal and applied to a digital signal processor (51) which is configured differently according to DSP running programs respectively stored in first and second memories for operations during the POTS and ISDN modes. The DSP processes signals received from the hybrids four-wire transmit port as well as from the telephone switching system according to the program supplied from the first or second memory and applies the process signals to the switching system and the four-wire receive port.