Abstract: The invention concerns exchanges switching telephone traffic, and in particular digital exchanges which have tie-lines passing therethrough. The exchange comprises control units (UC) and terminal units (UT) which are microprocessor controlled and which are connected to a switching network (RCX) by two-way multiplex links (MU). Some of the terminal units are connected to handle respective PCM junctions. At least one of the terminal units is a mixer and separator unit (UTB) which receives the signalling channels and in particular the signalling channels associated with the tie lines. This mixer and separator unit serves to forward signalling channel information for the tie lines. The terminal units connected to PCM junctions which include tie lines send the tie line signalling to the mixer and separator terminal unit, and also receive said signalling from said mixer and separator unit for onward transmission over a destination PCM junction.