Need a formula to update unformatted phone numbers?

Easy Fix : By default if the field you are trying to load the number is a phone field then the data is formatted automatically by salesforce.

As per docs : If you want to load the phone number to a phone number field and need to have your own formatting when you load the number add a + symbol if the your users/ BU allows that flexibility.

  https://help.salesforce.com/apex/HTViewHelpDoc?id=basics_understanding_values_curr_date_phone.htm&language=en_US

When you enter phone numbers in various phone fields, Salesforce preserves whatever phone number format you enter.

NOTE: However, if your Locale is set to English (United States) or English (Canada), ten-digit phone numbers and eleven-digit numbers that start with “1” are automatically formatted as (800) 555-1212 when you save the record