garbled text messages from international senders

I frequently receive text messages from friends in Honduras that look like this:
?H??i??? ?H??o??w? ?a??r??e??u? . . . It doesn't happen all the time and is not sender specific. Has anyone else had this problem? I have reset the phone multiple times, I "reset" the chip (removed it and put it back) - the problem still occurs. I have an iPhone 3G with vers. 3.0.1 installed. The carrier is AT&T in the U.S. I don't know the carrier in Honduras nor do I know if the garbled messages all come from only one Honduran carrier.