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

Don't use a numeric type for phone numbers, stick with a String.

Do some form of input validation. Define some restrictions on your phone numbers, and when reading the card, check the String against your format: if the phone number doesn't match your restrictions, then reject it.

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