In fact wasn't that the reason given for 999 originally instead of 111, as wires banging together in the wind could pulse (loop disconnect) dial these digits. Now we have tone dialling its a moot topic.
It is one reason and there were others.
On the old call boxes it was easy to make 9xx a free call alongside the 0 use to call the operator.
It was easy to locate the hole on a rotary dial - one away from the zero.
It was not susceptible to inadvertent contact during installation/repair work.
Other countries went for 112 and 911 to avoid the possible problems with accidental contact.
M H C
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