The Network Control Modem (NCM) was designed to monitor the 9-1-1 alternate answer circuit for line continuity and to signal the 9-1-1 tandem switch to re-direct 9-1-1 calls to an alternate PSAP. The NCM at the PSAP will send a heartbeat every six seconds to the NCM at the tandem. If the tandem does not receive the heartbeat then both NCMs will go into an alarm state producing both visual and audible alarms.

The NCM can also send a short along the alternate answer circuit to the 9-1-1 tandem switch where the scan point attached to the circuit is saturated. When this occurs the remote make busy feature is activated and the 9-1-1 calls associated with the circuit are re-routed to an alternate PSAP.