Why can't I get service on my iPhone 4?

For the past few days my service comes and goes. It has nothing to do with my area as everyone around still has service. I've tried turning it off and on. I've tried taking the sim out, I've tried turning it on and off of airplane mode. I restored my network settings, I restored All settings. I've run out of ideas. In the upper left corner it says "Searching… " or "No Service.". The first time this happened, it lasted for 17 hours! Then I regained service for a couple of days and then it went off again for about 7 hours. Now today it's been out for the past 9 hours. ANY IDEAS before I call AT&T tomorrow!

I would try a new sim card