Setting up Asterisk and E911

The Asterisk Open Source PBX provides a cost effective way of achieving E911 compatibility when used in conjunction with a telephone carrier that offers E911 database management services to its customers. This database management service consists of a reverse directory maintained by the customer, but residing with the carrier, that assigns location information to private (DID) telephone numbers that the customer uses.

In order for E911 to function on an Asterisk PBX, each extension phone must be assigned a 10-digit DID number. This DID number is then registered with the telephone carrier along with the location of the phone. An ISDN PRI link connects the Asterisk PBX to the carrier. This digital link utilizes a dedicated signaling channel (D-Channel) to handle call setup and signaling. It is the D-Channel that enables E911 functionality through the carrier. When a 911 call is placed on the PBX, Asterisk needs to be programmed to set the outbound CallerID (ANI) for the call to the registered DID for the calling extension. When the call reaches the Public Safety Answering Point (PSAP) this ANI number is matched with the DID number and location information which was pre-registered with the carrier. The exact location of the caller is then displayed to the emergency services operator.