iPhone 4 not syncing with iTunes

I have just recently started having trouble with my iPhone 4, it does have the newest OS. Just today it isn't allowing me to call out or receive calls, nor will it sync to iTunes....any ideas? Is there a downoad or reset that you might recommend? My service is A T & T, but I live in an area that is always off network. My family is on the same plan and all of their phones are working just fine. I would appreciate anyones help, thanks!