Where it all Started
Common Phone Keypad
Back in the old days, the first two numbers of a phone number
would indicate the exchange (actually, the "central office").
The caller would ask the operator to connect them to something
like "klondike-442", where the real phone number would be 55442,
with both the "K" and "L" filling in for a 5 each. See the
Name Project for the whole story.
Here is how the numbers are associated with letters:
2 ABC 6 MNO
3 DEF 7 PRS
4 GHI 8 TUV
5 JKL 9 WXY
If you look closely, you will see that it neither covers all
numbers, nor does it mention all letters.
Something Useful for 1 and 0
Officially, the numbers 1 and 0 did not map to anything on the phone pad.
This is because 1's and 0's used to be reserved for area codes. The middle
digit in an area code was always a 0 or a 1, and these two number could
not appear anywhere else in the area code so that the equipment used to
route phone connections could easily recognize area codes.
Even though these restrictions have long been lifted, none of todays
phone key pads (except for some European ones) associate the number 1
with any letters. The UK phone key is the only one to map letters to
the number 0. Still, some vanity numbers in the US, for instance,
incorporate the numbers 0 and 1 the same way some vanity license
plates incorprate digits.
What about "Q" and "Z"
Another Common Phone Keypad
The letters "Q" and "Z" have no numbers associated with them on most older
phone pads, but on many recent key pads, espicially on cell phones, they
are mapped to numbers.
There are actually many different types of key pads. Different
countries have different standards, but it is not uncommon
to find different types of key pads within the same country.
Certain scandinavian telephone key pads even find places for those
local funky looking squiggly characters.
However, most phone manufactured today use the international standard
ITU E 1.161, also known as ANSI T1.703-1995/1999 and ISO/IEC
9995-8:1994). It assigns PQRS to 7 and WXYZ to 9.