Using computed field to format phone numbers?
Try Masked Input module. It allows you to create a text field with mask that you can setup custom (e.g. 999.999.9999). When user enters info into the field with masked input, the information entered is formatted automatically and stored into database with that exact format. So the output of formatted information becomes that much easier.  

One drawback of this module is that it provides formatting only if user has JS enabled in the browser. If JS is disabled, you will rely on user to enter info in the desirable format. You can also ask the user to provide info in specific format by entering field description as a fallback (e.g. "Enter your phone number as follows - xxx.xxx.xxxx). 
I've used this module on websites with big user base, and I can tell that users very rarely have JS disabled when they fill up forms.