Twice recently I've hit the same problem with two different mobile phone vendor's websites. Vodafone (displayed here) and 3. When I type a phone number I split it into three sections using white-space. "nnnn nnn nnnn" that's how I remember numbers. That's not uncommon I don't think…

nor is it odd to use a dash.

So why do I need to learn how your website wants phone numbers formatted.

Whack some javascript on your page… you must be using it for something!

var correctedNumber = numberTypedOnForm.replace(" ","").replace("-","");

and with that massive development cost you aren't going to make someone type a number twice only to satisfy your database server. Yes, not everyone will have javascript turned on and it won't catch everyone's weird way of typing phone numbers

"(nnnn)-nn-nn-nn-n"

but it's about not introducing a pain point for customers when you don't have to

If you want to be really fancy you could

var correctedNumber = numberTypedOnForm.replace("/\D/g","");

Computers are supposed to make our lives easier but it's up to you website developers to help them help us.