Telephone system digital channel bank interface

8kHz ASK signals are produced by an electronic business set instrument to function as control or feature signals. To avoid the problems of transmiting the 8 kHz ASK signals over voiceband telephone lines over long distance, the least significant bit (LSB) of each 8 bit word representative of voice signals is cuased to be at a predetermined level (e.g., low) when an ASK signal is present and at the other level (e.g., high) when there is no ASK signal. A coder-decoder (CODEC) receives a composite signal in one transmission direction, the composite signal including voice signals and 8 kHz function code signals, and produces a sequence of digital words representing the voice signals for telephone path transmission. The presence of an ASK signal is detected and represented in a digital format which is inserted into the digital words. The CODEC receives digital words from the other end of the path, converts them back into annalog signals and generates ASK signals when the presence of such signals is indicated.