I'm having this same problem.
I called straight talk because I can't send or receive mms messages and the error message that pops up is saying that there is no network. Since I have owned the phone (1 month) I have yet to have full coverage anywhere. All of my other phones didn't have this problem. Tgogohe Tech Support person told me to go into my settings> mobile networks> unclick data enabled, reclick it, shut off phone then restart it....It worked for about 12 hours.....Now right back to this same issue. Did you ever get yours fixed?