In order for emergency services to be dispatched to a 9-1-1 caller, the caller must be located, the call must be routed to the nearest of over 6,500 Public Safety Answering Points (PSAPs), and the caller’s location must be automatically delivered to that PSAP. With traditional landline phone service, the phone company can easily determine the location of the caller, since it installed the physical phone line and set up the service at a specific address. This location information is stored in an Automatic Location Identifier (ALI) database. Information stored in the ALI database is used to provision selective routers, which route 9-1-1 calls to PSAPs, along with the caller’s phone number and address. This process is depicted in the call flow below.

Wireline E9-1-1 Call Flow

When cellular phones were introduced, their mobility caused some new challenges for locating the caller, routing the call to the closest PSAP, and automatically delivering the caller's location and callback number to the PSAP. Cellular carriers solved the location problem by using technology that identifies the location of the closest cell tower (“Phase I”) and by deploying technology like GPS into their networks so they can identify a caller’s latitude and longitude (“Phase II”). Cellular companies contracted with service bureaus to convert this location data into a format compatible with emergency services systems so that it could be automatically routed to the closest PSAP along with the call.

However, because selective routers were designed to work with landline phones, they don’t automatically recognize cell phone numbers; consequently, selective routers were not able to route cellular 9-1-1 calls. To solve this problem, cellular carriers created selective router-recognizable 10-digit numbers that could be temporarily associated with a cell phone number. Service bureaus then provisioned these numbers into the ALI databases and selective routers, so that cellular 9-1-1 calls could be routed to the correct PSAP. This approach has become the preferred solution for supporting E9-1-1 functionality in cellular networks and is depicted in the diagram below.