Rajesh,

You’ve probably considered this, but one way to do what you want is through a Salesforce validation rule. The only downside is that you’ll only see the error after the user saves. If you want to validate the number before the save, you have a few options. Check out this post to see how you can use a custom field renderer to notify your users of an error right after they edit the field; I think it might be just what you want. You could also consider using the jQuery mask plugin as discussed here. This community post also discusses custom field renderers for phone numbers, although I think it focuses on cleaning up the number entered.

Thanks @amy