vinsnash,
I'm not sure of your exact requirements, but an easy way would be to have 2 attributes:
1a- PhoneInput - text 10
1b- Phone - text 13
You can then make a rule:
Customer.Phone='('+SUBSTRING(Customer.PhoneInput,0,3)+')'+SUBSTRING(Customer.PhoneInput,3,6)+'-'+SUBSTRING(Customer.PhoneInput,6,10)
Hope this helps.
Tom