How can I prevent bad inputs when storing phone numbers as strings?

Question

I am creating a phonebook program that can read VCards. The problem is with phone numbers. Normally, they will be like this in the United States:

0771234560

Answer 1

What would adding two phone numbers mean? So, IMO, not a number.

You should still attempt to validate, but if you want to accept any and all arbitrary international numbers, including local conventions for how they're written, that'll be frustrating.