runssl

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.