Constantly getting no service on 3.0

[SaNcTuS]

Hello, I've had this problem for a while where my phone does not get service. At first it would only happen at work, I would either have no service while my co-worker with an ATT iPhone had full 3G bars. Then I would have full signal but nothing would work, couldn't make calls or send text messages.

The problem fixed itself suddenly it's been fine until today, my phone hasn't had signal in hours. I've tried restarting my phone, power cycling with no SIM card. Nothing seems to fix it. Is there something I could do? Is it time to take my phone to the apple store and get a replacement? Or is the problem with AT&T? Any help would be greatly appreciated.

[Cowboy]

If it is network it could be your sim card or the phone has a bad intena so id get a sim card first because that quick and easy if that doesnt work go to apple.