To be blunt, inputs you don't validate are inputs that will be entered wrong a significant portion of the time. Other concerns may override this for many fields, but unusable phone numbers are generally considered a problem.
If you force people to enter a phone number in a format they don't have, you will just end up with a bunch of 555-555-5555 numbers. Either way, you are going to get incorrect information.
It's much better to realise that your information will not always be 100% accurate no matter what you do.
The idea is to minimize incorrect information. A 555 number at least tells you something that a number in an unfamiliar format doesn't - whether this was intentional or mistyped.
I'm very much for allowing people to enter any format, but seeing how badly people mis-enter NANPA phone numbers, it is simply wrong to not acknowledge that you will see far more mistakes from an open input box.
Yes, but if you over-validate you also risk losing sales. What to do doesn't seem obvious at all, at least to me. You'd want to do some kind of cost-benefit analysis
Also, wouldn't a phone number in effect be validated for most online credit card purchases? I thought credit card processor often match the phone number you provide to the phone number on record for your account to prevent fraud. Skip the javascript validation, and let the cc processor validate the phone number.
...but I have no idea what I'm talking about, please correct me if I'm wrong.
If you under-validate, you risk sales you can't complete. Mind, that's less dire when you have email addresses - but we know the problems with trying to validate those on forms.
