You'll have to contact AT&T; anything else is just speculation.
You didn't mention which Internet Telephone Service Provider (ITSP) you are attempting to use - Google Voice, or some other provider (which one)? SIP control channel traffic is usually carried over the 5060-5080 range, but it can vary. Voice (media channel) can be carried over a variety of ports.
If you are using a generic SIP ITSP, like Callcentric,
voip.ms, or Anveo, then you can test whether the AT&T router is blocking traffic by installing a software phone client on your computer, and configuring it to use the ITSP. Counterpath's X-Lite or Zoiper are two good, free SIP softphone clients.