Have you tried putting an AT&T sim in the Razr Maxx? That's the only way to know for sure. You can find them cheap online or maybe the AT&T store will let you try one of theirs.

I am not sure what Elector is talking about with no carriers unlocking for US use until 2015. I'm currently using a Verizon iPhone 5S and a Verizon Galaxy S4 on an AT&T MVNO. Both were unlocked out of the box. The GS4 just doesn't get LTE which I knew going in.