Even if you want to stay on the AT&T network, why would you ever directly use AT&T service? U.S. mobile is an MVNO. I was paying more than $70 a line for AT&T's cr-ppiest unlimited plan.
With U.S. Mobile I pay less half that! I get all the same benefits including the same QCI, hotspot, a much better customer service experience, and I can switch to Verizon or TMo (and back) any time I want.
Or, you can literally get unlimited talk, text, and 2GB a month for $10 a month (!!!) INCLUDING taxes.
The only benefit I really thought was decent about AT&T was international day pass. But they jacked up the price on that and U.S. Mobile has a perfectly fine international roaming add on that's much cheaper than T.
There's NO reason at all to stay with T!