Even referring to them as “T9-letters” is a massive neologism. Those letters have been on telephones going all the way back to the very first rotary phones when you were first allowed to dial a number yourself without talking to an operator. Before, you’d tell the operator you wanted “Wabash 3 - 1234” and they’d connect you to number 1234 on the Wabash 3 exchange. To dial that same number when the dial telephone came around, you would dial WA3-1234, or 923-1234.
Here’s a great film from 1940 introducing people to the idea of dialing on a telephone and explaining how to use the letters: Internet Archive Link
The practice of using the T9-letters to memorise a phone number is just not done much outside the USA
Even referring to them as “T9-letters” is a massive neologism. Those letters have been on telephones going all the way back to the very first rotary phones when you were first allowed to dial a number yourself without talking to an operator. Before, you’d tell the operator you wanted “Wabash 3 - 1234” and they’d connect you to number 1234 on the Wabash 3 exchange. To dial that same number when the dial telephone came around, you would dial WA3-1234, or 923-1234.
Here’s a great film from 1940 introducing people to the idea of dialing on a telephone and explaining how to use the letters: Internet Archive Link
Yep I know about it from American media but never seen this done anywhere else that I know of (and I know how to type T9).