Iphone only get signal when connected to iTunes

So its a 3GS I bought a couple of days ago, and unfortunately updated it to 5.0.1 only to get horrible battery life and no service messages.

sometimes it does show some signal bars, but actually there is no reception (incoming calls go straight to voice mail, outgoing get no dialing tone and disconnected after a while).

I have already tried to reset, restore, replace sim, airplane mode, and parctically every solution I read here.

the only way I can make calls is when its connected to my PC with iTunes on.

Any ideas?.

thank you & kind regards.