Jan is right,
I have the following from In A Pickle,who used to work for BT.
The ONLY persons who could be connected to an XD number are as follows:~
Police
Fire
Ambulance
Coastguard
Doctor in Hospital( administering to critical patient)
and these ONLY in a matter of Life & Death.
The XD information is kept by a section of BT in (used to be) Aberystwyth.
The BT XD Operator in the call centre had to convince the XD Centre that the number being divulged was necessary.
Once the BT Operator had the number,they would call it and say "This is the BT XD Operator at(name of call centre) will you take a call from the Police" etc.
If the XD person agreed the BT Operator would call the Police etc and connect them.The ONLY person/s who knew the number were the XD centre,and the BT Operator in the exchange.
When the Police etc were billed for the call it appeared thus
01273 000000.
There were XD numbers which paid to be "accept calls",but Pickle tells me he never (in 25 years) came across one that needed to be.
ACtheTROLL
The Police often want to ring XD numbers in the case of car crashes,and other sudden bereavememts.
BTW,this all dealt with by the BT Operator on 100.