A ring scheduling apparatus for use in a digital multiplexed telephone communication system. The apparatus utilizes a controller which sorts messages indicating which telephone lines are being rung and what frequency and DC signal level are required to ring the lines. The messages are sorted into voltage and frequency compatible groups, and then digital data is sent to a ring signal generating circuit to control the frequency of the ring signal generated and the polarity of the DC signal upon which the ring signal is superimposed. Disclosure of a multiplexer and line interface circuitry for use in this system is included as well as a self-test circuit for testing the audio circuitry of idle telephone lines.