If there’s one thing I’ve learned from watching movies, it’s that the police need to keep the terrorist/kidnapper/psychopath on the line for a minute or two to get a trace on the number. Yet 911 seems to be able to trace a call instantly. I once dialed 911 instead of 411, and hung up when I realized what I’d done, before the operator even answered. Yet she called me back immediately to see what the problem was. I’ve always heard that once you’ve called 911 they can trace it and find you even if you’re only on for a few seconds. I realize this may not be true, but it seems to be the general consensus. If so, how? I can understand that maybe the 911 phones have had some kind of caller ID feature long before it was available to the general public, but this wouldn’t work with unlisted numbers or caller ID blocking. So can they actually do it (and how can they)? And if they can, why can’t the police use the same technology to trace calls?
I don’t remember what it’s called, but it is similar to caller ID except that it works even with blocked and unlisted numbers ( the information is still there with a blocked number-Caller ID just doesn’t disply it) and in some areas, it also displays the address associated with the phone number.
It is just 911/000? Because I thought Pizza Hut also had it - you know, for all the prank calls. Perhaps it’s some device built into phones? Sorry I can’t offer any more help than this. 
Caller ID and Advanced911/911Plus (I’ve heard it called both) are effects of the same cause, namely the automation of telephone switching. With computers running the switching, there’s no more mystery to tracing a phone call than there is to tracing an e-mail. The packet that arrives at the destination has the originating phone number attached to it the same way an e-mail has the originating computer’s IP address attached to it.
The trick is to have the database that the 911 dispatchers have of addresses to go with those phone numbers, which is why Advanced911/911Plus costs more. Normally, only the phone company has that database and if I understand it correctly, they still own it even when they give access to the 911 folks to use it.
More advanced 911 will be able to triangulate a location of a cell phone call based on what towers it’s using to broadcast, eliminating the need for cell users calling 911 to give their location.
One thing to keep in mind is that a 911 call naturally goes through only the local exchange. That’s always going to be easier to trace than a long-distance or international call …
For several decades now, most U.S. phone companies have been using an internal technology called ANI – Advanced Number Identification. It predates the consumer-grade Caller-ID, but cannot be controlled, blocked, viewed or detected by the caller. This is the technology that made a serious dent in hacking from home – if every call that goes out from your phone has the source number imbedded and logged, you might not want to run a war-dialer overnight to test a bunch of numbers for modems!
So modern calls do not have to be “traced” in the movie-style time-consuming process. Once the call is made, ANI has already tagged it with the data the police want.
You cannot block your standard Caller-ID number, either, for at least two special cases I know of – 911 and 800/888 area code calls. The 911 case is for safety, of course, but the 800/888 calls prevent blocking on the theory that if the recipient of the call is paying for it, they are entitled to know who is calling. And unlisting your number will have no effect.
Musciat is right, 911 centers use ANI. Also, 911 calls all go through the local switch, so there is no inter-switch checking that needs to be done.
I might be wrong about what ANI stands for – I’m not sure if it is "Automatic - " or "Advanced - " Number Identification. Either way, it makes being a phone phreak a lot harder than it usta be.
When dealing with an Enhanced 911 (aka E-911) system, The Automatic Number Identification is matched with an Automatic Location Identification (ALI) database. Way back when I sat through my dispatching classes, we were told that as soon as you hit the second “1”, the call is sent to a “local” exchange (in a city about 35 miles away from here in SE Massachusetts, not quite as local as it sounds), is matched with the ANI, and is then routed to Syracuse, NY to be matched with the ALI, then its bounced back to the local Public Safety Answering Point (PSAP) where the call is physically answered by a 911 calltaker. The 911 calltaker will then bounce your call to who/where it belongs (police, fire, or medical). Thats how it works here, your results may differ.
In our fire alarm office, there is a monitor and a printer that spit out the name, number, and address of the 911 call. The odd thing is that the monitor comes on and the printer starts to print about a second before the phone rings. Tell me thats not freaky.