Comment by iagovar

They were generally present beginning in the 1980s. The phone you linked is from the 1960s. The layout with Q and Z was standardized by ITU-T in 1988 https://en.wikipedia.org/wiki/E.161

Earlier in time the letter "O" was also not generally on phone keypads.

the Q & Z assignments shown are typical. That said, few text-based phone numbers include these characters, for the obvious reason.

Depends, I used to have one of these: https://www.geekzone.co.nz/forums.asp?forumid=39&topicid=114...

Though the chart is still wrong according to that, however it (the chart) does match what my Android phone's dialer looks like.

You're correct, which appropriately enough is why you're downvoted. The letters were placed on telephone dials when telephone ‘numbers’ used a mnemonic exchange+digits format¹, so there was no requirement to include the entire alphabet. Advertising mnemonics like the one in the grandparent comment came later, and entering arbitrary text such as names, which actually requires the whole alphabet, much later.

¹ https://en.m.wikipedia.org/wiki/Telephone_exchange_names

It's not a "custom". The reason keypads have letters is precisely because the letters are mnemonics for the real number. This usage predates mobile phones, and is not US-specific.

They were originally used for area codes (Wikipedia lists a UK example of 0AY6, ie 0296, for Aylesbury), then later for mnemonic numbers like the one from the article. Mobile phones inherited the lettered keypad from landlines and also started using it for typing text messages.

You're thinking of T9, which is for typing text. When you dial 800-AAAAAAA, you're not typing, you're dialing. The "A" character is just on the same button as the "2". So when you press "A" seven times you're really pressing "2" seven times. You end up dialing 800-2222222. There's nothing funny happening, "A" is just another symbol printed on the "2" button.

Mobile phones took this cue from telephones, which have had this kind of notion of an associated set of alphabetic characters since at least the eighties or nineties.