Back in those days when mobile phones had physical keyboard you had to press the same key multiple times to get a letter and this is where the old mobile keyboard code originates.
The button layout of those phones looked like this:
So, for example, to get letter N you had to press key 6 two times. So, it could be written like this N = 66.
So, A = 2, B = 22, C = 222 and so on.