I bought an iPhone 4 two days ago, and I hadn't had problems with text messaging until I tried sending a picture message today. Since then, I haven't been able to send or receive texts. I've checked my settings to make sure MMS was enabled, and it is. I've turned my phone on and off and I've done the reboot. It always shows that I have full reception, and text messaging is still not working. What more can I do or try in order to fix this problem?