When I was little we used to have a rotary phone (the type where you turn the dial around to dial each number). I believe it was green. Anyway, along with the numbers it also had letters assigned to each number.
This was a throwback to when instead of having area codes that were numbers, they'd have an area code made up of letters. You'd dial the letters of the town the phone number was located, and then the rest of the phone number. Later, the letters were replaced with the numbers, and phone numbers began to look more like they do today. (So my phone number for London began 08, which is odd, because the L isn't on the 8 key, but London has to do things differently!)
Anyway, when I was little, we were past the stage that the letters were being used, and I used to look at my phone and wonder why on earth they would bother even putting the letters on there anymore.
Today, as I was texting HP, I realised how fast things things have changed. In the past 20 odd years, telephones have been redefined. People definitely use the letters on phone keypads once again. The circle is complete.
As an extra aside, I often wonder what small children today make of toy rotary phones. We have one, and I know that that Fisher Price telephone is a common toy at playgroups and mum&tots groups, and yet in reality hardly anyone has a rotary telephone anymore.