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What Happens When I Call 911?


What is Basic 9-1-1 Service?

Basic 9-1-1 service is an emergency telephone system which automatically connects 9-1-1 callers to a designated 9-1-1 Communications Center. Basic 9-1-1 can provide ANI information for the caller if Caller ID is enabled on the system.  For wireless calls, the handset must have battery power and be in a coverage area to complete a 9-1-1 call.

What is Enhanced 9-1-1 Phase II service?

What is Enhanced 9-1-1 Phase I service?

Enhanced 9-1-1 Phase I service was the first step in providing better emergency response service to wireless 9-1-1 callers. When a wireless 9-1-1 call comes into the 9-1-1 Communications Center (also called a Public Safety Answering Point or PSAP), the call-taker's computer screen will show the wireless phone's 10-digit call-back number if one has been assigned to the calling handset. In the event the wireless phone call is dropped, the call-taker may contact the caller. Phase I service also identifies the cell site/sector location information that the call was made on to the call-taker's computer screen. This provides the call-taker with a general location (i.e. within the site coverage area) of the caller.

What is Next Generation (NG) 911?

Because most 911 systems were originally built using analog rather than digital technologies, and were voice centric systems, they do not fit today’s methods of communication.  Public safety answering points (PSAPs) across the country have been upgrading to a digital, Internet Protocol (IP)-based 911 system, commonly referred to as Next Generation 911 (NG911). The success and reliability of 911 will be greatly improved with the implementation of NG911, as it will enhance emergency number services to create a faster, more resilient system that allows voice, photos, videos and text messages to flow seamlessly from the public to the 911 network. NG911 will also improve PSAP ability to help manage call overload, natural disasters, and transferring of 911 calls and proper jurisdictional responses based on location tracking.

Most modern PSAP’s are equipped with Enhanced 911 (E911), which provides specific information about the 9-1-1 caller.  When a person dials 9-1-1, their call is routed to a selective router, which matches their phone number with a database record that identifies the Public Safety Answering Point (PSAP) that services the callers address.  The database record includes the caller’s phone number (Automatic Number Identification or ANI), and the address associated with the telephone number (Automatic Location Identification or ALI).  The database record also contains the subscriber name associated with the phone number, the telephone service provider information, and the class of service (whether it’s a cell phone, business phone, pay phone, residential phone, etc.).  The ANI and ALI information is presented to the 9-1-1 Operator when the call is completed at the PSAP.


Cell phones do not provide addresses, since they are portable by nature.  Wireless Phase I and Phase II service allows 9-1-1 Operators to get an idea of a cell phone user’s location when they dial 9-1-1.  Wireless Phase I service provides the location of the cellular tower that was utilized to complete the 9-1-1 call.  Cellular tower antennae arrays are comprised of multiple tower “faces”, which send and receive radio signals.  Tower faces are assigned to the PSAP that serves the majority of the area covered by the tower face.  Pseudo telephone numbers (P-ANI) are assigned to these tower faces and when a 9-1-1 call is received on a tower face, the P-ANI is attached to the call before it is delivered to the selective router.  The selective router then utilizes the P-ANI to retrieve a database record that identifies the PSAP that should receive the call.  When the call is initially delivered to the 9-1-1 Center, the ANI/ALI information provided to the Operator will be the information relating to the tower site.  Once the call is connected, a process called rebidding is completed.  This process sends a request back through the telephone network requesting updated location information from the wireless provider.  The provider’s use either handset based technology or an algorithm based location technology from tower sites to provide an estimate of the location of the wireless device.  This process is known as Wireless Phase II.  The location of the device is provided as latitude and longitude coordinates that are plotted on a digital map in the PSAP.  The accuracy of the location information is generally between 50 and 100 meters of the actual location of the caller.

With Phase II service, the location information that is provided to the 9-1-1 Communications Center is the approximate X, Y (longitude, latitude) location of the handset making the call. This location information is typically more accurate than the Phase I location information (cell site/sector). As with Phase I, Phase II service allows call takers to receive both the caller's wireless phone number and their location information.

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