When anyone dials 911 in an emergency, they expect to get to the correct 911 center to be able to send the proper response to help. When the 911 service started, all phones were connected by wire to a central switching station. The phone company knew the exact location where the phones were located, so when 911 was dialed it would know the proper destination of the call. As the networks improved technology, more information was sent to the 911 center with the call, including the address of the caller, making it easier and more accurate to dispatch resources.
With the advent of mobile phones and VOIP (Voice Over Internet Protocol) technologies, knowing the exact location of the caller became more difficult, a problem for both the caller and the 911 center. Enter E911.
E911 was developed for VOIP so that a caller using the service can specify the exact location they are at and have the call routed to the correct 911 center for the area. The location information is relayed with the call to the 911 center.
Another unique aspect of E911 is the callback. From many VOIP systems, there are call routing devices that create a roadblock for calling back a caller to get more information. Generally, the Caller ID phone number sent with the initial call is a number that is shared by an entire company or large group, and calling it back can take time that is not available in an emergency. E911 makes a link between the SIP address of the caller and a temporary phone number, so if the E911 center or responders need to call back, they can call directly to the phone that made the call without having to lose time searching through an attendant or someone else answering the call.