when i worked for good old BT.I was told the reason its 999 is that no UK number started with a 9. so even if you managed to only do 99 you call would still go through after a few seconds even on the old analog switching system.
if you had a silent 999 call you asked the caller for a 55 keypress or nay sound. If you got anything you put the call through to the police. Even if you thought you might have heard something.
even in the days of analog mobile phones we knew which cell tower the call was connected to. rumour was dire to the many antennae's on the mast and signal diveristy they would know which direction. that bit could only be done by cell company