> If you don't use a skill, that smo atrophies

I don't think the _ability_ to memorize phone numbers is what atrophied here by getting a cell phone. The _need_ did. It's harder to memorize phone numbers today because we lack the practical application of the memory (e.g. only when something fails do we actually try to put that memory to practice). I bet anyone would quickly pick up on memorizing phone numbers if they had to both remember and type in the phone numbers every time they wanted to make a call or text.