If the users want to give you their phone numbers, then trust them to get it right. If they do not want to give it to you then forcing them to enter a valid number will either send them to a competitor’s site or make them enter a random string that fits your regex. I might even be tempted to look up the number of a premium rate horoscope hotline and enter that instead.
I would also consider any of the following as valid entries on a web site:
"123 456 7890 until 6pm, then 098 765 4321" "123 456 7890 or try my mobile on 098 765 4321" "ex-directory - mind your own business"