Abstract: In a telephone system for digitalized speech transmission additional digital information is transferred without disturbing a speech communication in progress. At the sending side, time slots for the transmission of the additional digital information are created by momentarily suppressing the digital speech signals transmitted on a digital channel and, at the receiving side, the digital information is extracted from the time slots and replaced by digital signals which constitute a continuous amplitude function starting from the amplitude value which was transmitted immediately before the time slot. To distinguish the additional digital information from speech signals a special identifying code is added at the sending side. The speech transmission may employ either Delta Modulation or Pulse Code Modulation.