The thing I can never work out is why, when those phones were ubiquitous, did they make the UK emergency service number 999, which by my reckoning is the 4th most time consuming 3 digit number you can dial?
Why not make it 111?
I remember someone telling me a long time ago that in the old days, telephone lines could trigger a signal through close (if not complete) contact with each other, with a run of consecutive contacts corresponding to its respective dialled number on the telephone set (e.g. the dull noise pips you may hear over the earpiece when dialling out a number). If the emergency number was something like "111" then it would trigger too many false alarms too easily simply by having adjacent lines rubbing against each other by accident, so a sufficiently large number was chosen instead, hence 999.
I found this article on the subject too: