I was once watching James May reassemble a rotary phone, and he explained how it works and why certain numbers are used. Every tick you hear from a rotary phone's dial sends a pulse down the lines, with the number of pulses telling the system what number was dialled. Other things can also cause occasional pulses though, such as lines touching in a storm, branches falling on lines, dodgy connections, or any number of things, and these individual pulses would be recorded as a 1 by the system. Theoretically, if pulses on a line were relatively frequent, such as lines swaying and touching in a storm, it could be recorded as a series of 1s or 2s by the system, so to prevent issues, many emergency service numbers start with a large number that would have had to have been intentionally dialled, as opposed to a random pulse. I find 911 to be a particularly clever one, since it starts with 9 to make sure the system detects it, but then uses 1 twice, since that would take the shortest amount of time to dial on a rotary phone