How A 9-1-1 Call Works

1.    A caller dials 9-1-1.

2.   The end office serving that caller will recognize the call as a 9-1-1 call and send the call, via a dedicated 9-1-1 trunk, to the tandem central office. The end office will also send automatic number identification (ANI) (caller’s billing telephone number) with the call.

3.    The tandem office recognizes that it is receiving a 9-1-1 call and looks for the automatic number identification and finds the associated emergency service number (ESN). The emergency service number assigned to each telephone number in the tandem office is dictated by the address of the caller. The emergency service number points to a seven-digit number associated with a particular Public Safety Answering Point (PSAP).

4.   The Public Safety Answering Point automatic number identification equipment sees the 9-1-1 call and sends a signal back to the tandem office to forward the automatic number identification. When the Public Safety Answering Point operator answers the ring at the Public Safety Answering Point position, the automatic number identification is displayed.

5.   The Public Safety Answering Point automatic location identification (ALI) equipment then sends the automatic number identification to the E9-1-1 host computer to retrieve a lookup on the address associated with the displayed automatic number identification.

6.   This retrieval request goes to the host computer via two data circuits and the address record is forwarded back to the Public Safety Answering Point.

7.   The Public Safety Answering Point operator sees the display of the automatic number identification (address information) and continues to process the emergency call. The automatic number identification and automatic location identification will remain displayed until the Public Safety Answering Point operator terminates the call. At the termination of the call, a printer at the Public Safety Answering Point will log the time the call came in, the time it was answered, the position it was answered on, the time the call was transferred (if applicable), the time the call was terminated and the trunk number the call used.