If you don't use a skill, that skill 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.
Of course, this is likely totally a regular memory device people would use. I've done no reading on this so, so if Socrates was wary of writing, was he also wary of pattern recognition or drawing as memory aids?
I don't even know my partner's number, or email address...
But what if it isn't? Like, if you didn't have the written word to rely on as a crutch, what more could you have accomplished if you didn't have that? Maybe nothing, but maybe much, much more! Who can say in retrospect?
I've always wondered how we'd fare in battle against an extrasolar civilization which achieved technology without writing. They'd be a hell of a lot scarier than we are, in my view.