ok, from memory of my days in the emergency operator section in bt, all 999 calls, from non-mobile phones and from anywhere in the UK (not Dublin) are answered by a dedicated, emergency, BT operator. On the screen, it displays the telephone number of the caller and their house address. The caller tells the operator what service they require and the bt operator connects the call to either the fire, police ambulance or coastguard service, as requested. The operators screen will tell him/her which area fire/ambulance/police/coastguard to connect to, based on the phone number. If the person calling the operator does not speak or puts the phone down, the operator passes the details to the Police who will send an officer to investigate (and if no one is in, they will force entry on the basis that the person who made the call has collapsed or is in danger).
The same applies to calls from a mobile phone except that often the address of the caller is not displayed (if unregistered pay as you go) and also because if it is a mobile you are probably not at home anyway! The cell data is displayed though and will tell the operator where you are making the call from - this enables the call to be passed to the appropriate regional fir/police/ambulance coast guard office.
Hope this clarifies the situation!