Abstract: A pulse code modulation (PCM) time division multiplex (TDM) telephone exchange to which n telephone lines are sequentially connected during n respective sequential time slots. An unlimited number of phones may be connected together for conference purposes by means of a summation memory in which each conference storage location is used for accumulating the sum of message samples from one or more lines during a write-in cycle and supplying the sum of the message samples to one or more of the lines during a subsequent read-out cycle. The summation memory is indirectly addressed by a connection control memory. A portion of each message sample stored in a sample memory is subtracted from each sum of message samples read out during the same time slot from the summation memory, so that a speaker hears an attenuated sidetone version of his own voice.