It's kind of like mixing apples and oranges.

North American phone numbers are longer than in other places because other places have a country code, while a lot of NA uses a single code, +1.

This causes the problem of having to fit all those numbers under said code. Which makes the numhers themselves longer.

In the days before smartphones, people had to carry a notebook with numbers or just remeber them, so someone at Bell Labs git the idea to print letters on the number pad of phones.

What this does is make it easier to remember - for example, instead of remembering to dial 18002274846466 you dial 1800ACTIVISION.

For this you'd just press the key with the letter on it once. The phone line doesn't use numbers or letters, but electrical signals. These signals correspond to the button pressed. So instead of calling it the "Top left button", etc. it was labeled as "1". Then ABC was added, but the idea was the same - you press the button with the right number/letter on it.

Similarily, if you formst a number with spaces and dashes - you don't dial those. They're likewise merely a tool to aid memorizing information.

SMS was a newer invention. You had these number pads with 12 buttons, labeled with numbers and letters. However, now you wanted to actually differentiate the different letters from numbers and from each other. You could just press the number once, but then the person on the other end would need to decode that "439” is "HEY" and not, say "IDY". The simplest way was to make it so you'd need to press the button multiple times.

In essence, people first came up with the idea to add letters to phone keys to aid in memorizing numbers - however, it was still the number you dialed, not an alphanumeric code. Only later did the need to be able to specify a letter come. You use the newer convention for texting, and the older one for calling.