TracFone (Straight Talk, Net10, etc.) has their own roaming agreements. This is why their coverage map for GSM-T (T-Mobile) is more attractive looking than T-Mobile's own prepaid coverage map.

So, you're basically getting AT&T prepaid coverage supplemented by TracFone's roaming partners. Coverage on Net10 should be better than AT&T prepaid.
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-Rob Riccardelli
80GB 16MB MK2 090000736