Abstract: Digital telephone apparatus suitable for use as a subscriber station as well as for an attendant console. The telephone apparatus is connectable for full duplex communication with a four wire telephone transmission line. The telephone apparatus is capable of transmitting PCM speech data and/or digital data from a digital data interface to a peripheral data system. The telephone apparatus comprises an internal data bus to which are connected a digital transceiver, a sync/signalling generator, a serial frame synchronizer and at least one codec/PCM filter. The transceiver transmits and receives digital data to and from the telephone transmission line. The sync/signalling generator repeatedly supplies an 8 bit first data word to the internal data bus containing a binary synchronizing code and a signalling code. Simultaneously, the serial frame synchronizer repeatedly receives from the data bus and decodes an 8 bit first data word containing a synchronizing code and a signalling code.