I have an 8gb iPhone 2G. It is jailbroken, unlocked, and I am using a prepaid AT&T SIM card. It has been working fine until a couple days ago it began moving sluggishly and not receiving any text messages or calls. People would call and text me and the phone would not notify me whatsoever. The odd thing was, as soon as I took the SIM card out the phone began working like it was brand new, no sluggishness or anything. What can I do to fix this? I really need to be able to get texts and calls....D:. Thanks in advanced.