If your ATT phone all of a sudden won't allow you to make or receive calls, says "emergency calls only" or something similar, there is an "old school" method that will more than likely fix it:
All you have to do, is turn your phone off, let everything power all the way down, and then turn your phone back on. This forces any updates to come through and the majority of the time, will allow your phone to begin working normally again.
It's not mentioned very often, but it's recommended that you turn your phones off at least once a week to keep everything updated. Try this remedy first before you call customer care or come to a store.