Phone number validation.

I am new to whmcs and have searched for hours on how to validate the phone number field. I see a allot of people that have been looking for the same thing but never found a answer.

How can I make the user registration "phone number" field only contain numbers.
EX. (444)123-2233 would be saved as 4441232233.

If this is not possible where do I change the text only on the registration page to say something like "Numbers Only" in red.

Thanks in advance for your help.