> The Australian letter-to-number mapping was A=1, B=2, F=3, J=4, L=5, M=6, U=7, W=8, X=9, Y=0

wat?

> In the United Kingdom the letter "O" was combined with the digit "0" rather than "6".

nooo...

> In Norway, the North American system with the number '1' corresponding to one pulse was used, except for the capital, Oslo, which used the same "inverse" system as in New Zealand.

holy crap

> the United Kingdom selected 999 due to the ease of converting call office dials to make free calls. "0" for the Operator was already free, and the cam that removed the shunt on the line when the dial was rotated to the "0" position could be altered to include the adjacent digit "9" (and "8" if required) so that calls to "0" and "999" could be made without inserting coins.

And there we have the explanation for GPs assertation that "999" was the best number, it was instead a purely technical limitation in the mechanical construction of pay phones.