Comment by ErroneousBosh

In the UK, it happens when you call 999 or 112. I don't think 911 is supported, although it probably should be (it'd be a mess to get everyone to agree to add it to their routing tables, but I bet there's a nonzero proportion of people who watch American TV programmes and think the emergency number is 911 - or, for that matter, American tourists).

When you dial 999 it forwards your phone's GPS location if it has a lock to the provider, who then forwards it on to one of the 999 call handling centres in the UK, who then in turn forward that on to the appropriate emergency service control room. All the various services use various different products for telephony and dispatch but they will show the incoming location, and often will prepopulate an incident with the location.

The system that does this is called "EISEC" - Enhanced Information Service for Emergency Calls - and has a lot of cool stuff defined in the spec (which is publically available! You can just go and read it! BT offer a "Supplier's Information Note" with the protocol and details of how the information is encoded) that also handles calls from landlines. These are easy - your telephone provider knows where you live. OMG! The phone company know where I live? Yes, dumbass, they pulled a wire right into your house, of course they know where it is. For VoIP the situation is a little different but you can notify your VoIP provider of the location that the number is being used at, and it'll inject that into the EISEC request.

You can do other cool stuff like if you've got fixed mobile telephone in a vehicle, you can assign the make, model, registration number, colour, and so on in the EISEC database, so given a call from a phone number they know what car they're looking for. No-one uses this.

The very great majority of calls coming in to 999 are from mobiles. It's extremely rare to get one from a landline.

None of the providers use triangulation for determining where a phone is, it's all GPS.