Adds two variable ISDN ports (external/internal). When used as internal ports, either S0 (4-wire) or UP0 (2-wire) operation is possible. You can connect COMfortel system telephones or any other ISDN terminal device to the internal ISDN port. Number of modules which can be used in the COMpact 4000: 2 Number of modules which can be used in the COMpact 5200(R)/5500R: 5