Abstract: Two-way telephone communication is established between a subscriber-dwelling and a remote station using the automatic dialer of existing security apparatus. A speaker and a microphone are placed in the dwelling. A diplexer electrically coupled to the automatic dialer and responsive to a tone of a given frequency, operates a switch which selectively applies either the microphone or the speaker to the audio circuitry of the dialer. When the dialer is triggered, the switch couples the microphone to the audio circuitry of the dialer whereby sounds in the dwelling are communicated to the remote station. When an employee at the remote station applies the tone to the link, the diplexer causes the switch electrically to isolate the microphone and couple the speaker to the dialer whereby the employee of the station can communicate with the dwelling. Alternate operation of either the speaker or microphone prevents accoustical feedback between the two.