I'm using a unlocked Huawei E-220 modem with either a valid T-Mobile Pay-as-You-Go SIM or a ATT GSM SIM (iPhone contract) and on both Mac and Windows XP machines I'm getting the "SIM card invalid. GSM network registration failed." message with FrontlineSMS.
Yes, both SIM chips are live, activated and tested. For the E-220 I downloaded the Mac software and the XP machine auto-discovered it. Both computers recognize the modem and try to dial out on it to connect with T-Mobile or ATT as a data modem (don't have contract for that, so this operation fails), so I know the computers are recognizing the modem.
Any help would be appreciated.