Communication system with ISDN as a backup of inter-PBX tie trunk

In a private branch exchange, PBX line interfaces, a tie trunk interface interfacing a distant PBX, an ISDN interface interfacing a public switched ISDN network, and speech codecs are connected to a switch. Each speech codec provides data-rate conversion between a 64-kbps signal and a lower bit-rate signal. A mux/demux unit is connected between the codecs and the switch for multiplexing the lower bit rate signals from the codecs into a multiplex signal and demultiplexing a multiplex signal into lower bit rate signals for coupling to the codecs, respectively. The switch is controlled in response to a call from a PBX telephone to establish a connection between a line interface and any of the codecs and a connection between the mux/demux unit and the tie trunk interface when the distant PBX is the destination. If the call is destined to an ISDN user, a connection is established between the line interface and the ISDN interface. Should the tie trunk fail, a backup connection is established between the mux/demux unit and the ISDN interface and the ISDN network is signalled to establish a connection between the ISDN interface and the distant PBX.