Aren't I lucky?? Having the SAME ISSUE. My service has been terrible in which I call and complain what seems like EVERY month. In the past they claim they are servicing the tower in my area and there is nothing they can do resulting in the crediting of my account. Most recently however, although I was told the same thing, I was transferred to tech support who did something to my phone on his end and my data started working again.... in 5 minute bursts. Roughly 20 minutes ago an error message pops up stating SIM Card locked power cycle and its persists contact customer service. I can't wait till this 2 year contract is up.
take your phone into a repair store. they can test with a different sim card to see if that is the problem. it could also be the sim card tray itself but they will need to test this stuff to figure out what the cause is.