Patent US 3,989,901

An optional telephone answering system is connected in parallel with the telephone of a subscriber. When the subscriber does not answer a call the system is operated. At first an announcement is delivered to the caller telling him if he wishes to leave a recorded message he should speak after hearing a first tone but if he wishes to speak to a line operator he should wait to hear a second tone and then request the operator. At the end of the announcement a recording unit is energized. If the caller then starts talking the recording unit stays energized recording the caller's voice signals. The recording unit is deenergized at the end of the message. If the caller did not start talking a second tone is generated a given time after the first tone and the recording unit is deenergized. When the caller now gives an indication by voice or dial signaling, the answer service operator is called onto the line to take the message.