I just ordered my OP3. I would like to take advantage of the 2nd SIM slot. I'm on T-Mobile and I'm assuming that the OP3 only works on GSM carriers. Does AT&T or a MVNO of AT&T have a prepaid sim that I can just load and use when needed incase I get stuck in an area where T-Mo doesn't work? I don't want to add another monthly bill.