There is a standard, universal mapping of letters onto numbers. This allows companies to advertise their phone number as something memorable like 1-800-CASH-4-AU and everyone knows how to dial it. People don't even need to remember the mapping of letters, because they're printed on every keypad.
In the US, phone numbers started with 2 letters well into the 1960s. They were usually the first 2 letters of the street name the nearest telephone exchange was on. My grandfather's address book was full of numbers still written that way.
The numbers all have three or four letters above them and people (mostly businesses, really) will sometimes get a number that spells out a memorable mnemonic. That's all he's talking about.
Well there are the A B C and D DTMF tones not sure if those technically count as numbers and from memory they are without the ITU spec for a phone numbers.
http://www.pachd.com/free-images/technology-images/phone-dia...
There is a standard, universal mapping of letters onto numbers. This allows companies to advertise their phone number as something memorable like 1-800-CASH-4-AU and everyone knows how to dial it. People don't even need to remember the mapping of letters, because they're printed on every keypad.