If you find an old phone with a physical keypad, you'll find that the numbers 2 through 9 have letters on them as well. 2 can be an A, B or C. It means you can choose a phone number that is easier to remember because you'll dial it if you spell a short phrase. 1800-392-6753 feels random, but 1800-EXAMPLE doesn't. There's a whole bunch of us who have fond memories of 1800-465-3847. Better known as 1800-GOLF-TIP.

I think the British area code system involved this at some point, like the area code for Aberdeen was 22 for AB, though they deprecated that ages ago.

Its a word you can spell using the valid letters in the T9 keyboard format.

Back in the day, cellphones only had number keys, and the alphabet was split up to put 3-4 letters on each number (when texting, you would rapidly press the number to cycle though the digits assigned to each button.

So, when doing a phone number with an easy to remember phrase (eg 512-fuckyou), you would press the number key that the letter was assigned to. Because it was a phone call, which only numbers are valid for, you would only press the number key once, instead of cycling through the available digits on that key, which turns fuckyou into 3625968.