It is a known fact that generally MOST mvnos of Att have sims that work in locked Att phones, AT LEAST for talk/text. Data /mms may be trickier. I'd say Red Pocket, Air Voice, Jolt mobile and Pure prepaid/talk sims will work in locked Att phones.
This happens with Tmobile too and their mvnos and even Nextel with Boost mobile (iden/walkie talkie) . The sim of the prepaid division/mvno will work on phones that are locked to the parent network.
With limitations but talk/text almost always has no issue.
The OTHER way around, Att sims in mvno phones isn't always universal. H20 phones used to not allow Att sims on their phones without unlocking. Whether this is still true or not it is unknown. Tuyo, a Tmobile mvno does not allow Tmobile sims to work on their phones.
Tuyo though is still about two years behind the Att mvnos and other Tmobile mvnos (no unlimited, no data,no mms, no heavy user plans. Just paygo.
Tracfone, Net10 and Straight Talk all run Att and Tmobile networks on their coverage but their sims are completely locked down with the occasional rare exception (Nokia e71 sim works in unlocked gsm phones).