I took a while for me to figure it out. Go to Settings>Access point names (APN) Then go down the list and choose the first setting named "SPCS Global" That made it finally work for me. There's a lot of discussion on xda forums about the technical specifics of APN's. That's were the problem seems to be. I had to go down the list of APN's one by one until I got that one to work. So far, so good, but I'm still in testing mode to see if I'll keep using it. To be honest, I feel all the problems are related to the fact that we are on Android JB 4.1. I bet you when we get updated to 4.3, the problems will resolve.