If you want to Hangout on your AT&T Android phone, you will probably need a wifi connection.

AT&T Customers Will Probably Have To Wait Before Using Google Hangouts Over Cellular

If you’ve got an Android phone, you might have noticed that your pre-loaded Google Talk app has recently been updated and is now “Hangouts,” the name of Google’s video chat service. But if you’re an AT&T customer, you might have to wait a while before you’re allowed to use that app over the carrier’s wireless network. [More]