Make a backup of your file as a safety.
1. If necessary, find or omit the records in the database file to show only the records with phone numbers you want to modify.
2. In the current record, select the contents of the field you want to replace. ( your phone # field)
3. select Records Menu> Replace Field Contents.
4. the replace field contents dialog opens.
5. select the third radio button option - Replace with calculated result.
6. enter the calculation - "0"&yourPhoneNumberField
7 Hit OK
8. Hit Replace