According to Wikipedia: > The 9-9-9 format was chosen based on the 'button A' and 'button B' design of pre-payment coin-operated public payphones in wide use (

According to Wikipedia: > The 9-9-9 format was chosen based on the 'button A' and 'button B' design of pre-payment coin-operated public payphones in wide use (first introduced in 1925) which could be easily modified to allow free use of the 9 digit on the rotary dial in addition to the 0 digit (then used to call the operator), without allowing free use of numbers involving other digits There's a citation, but it's a book from 1950, so not particularly easy to verify. https://en.wikipedia.org/wiki/999_(emergency_telephone_number)#History https://en.wikipedia.org/wiki/999_(emergency_telephone_numbe...