In a security system the normal way to prevent the dialer from being confused by an answering machine or other phone set in your house being off-hook is to have the telephone connection flow THROUGH the security device. The phone line at your incoming point goes directly to the security curcuit board. Wire the two sides of the phone line to the C (common) terminals of a DPDT relay. (select a relay that is designed for TELCOM specs and application). Then the two NC (normally closed) terminals of the relay are connected off to the telephone wiring network in your house. The NO (normally open) contacts of the relay connect to the DAA circuit of your security system. When the security system wants to make the intrusion calls it activates this relay so as to disconnect the rest of the house from the phone line. It also gives the security system free access to the line in the case that an intruder has taken a phone set off-hook.

Michael Karas