To expand on Chas. and Domchi's points: There are two reasons to validate a phone number:
- To help the user to correctly enter his/her number (often when the user WANTS you to have their number)
- To prevent the user from proceeding without entering a valid number (often when the user would rather not give it to you)
If your case is case #1 simple validation is all you need since you don't want to prevent a fringe case correct entry and frustrate your user, you simply want to help them from incorrectly entering their number.
If your case is case #2, no amount of validation will prevent a user from entering a valid number that isn't theirs. To really do this you would need to do an automated call-back with a validation code, in which case you are now more in case #1 since they will need to provide a correct number to get the validation code. This will be MUCH more work though.
In the end, you really only need to cover case #1, help the user to enter a correct number. Don't frustrate away a potential user, just due to phone number validation.