I have tried to activate the iPhone with non AT&T SIM Card using iAsign and noted that despite of the fact that I cound not make calls, the SIM was somehow registered at the network. I was able to call to my number and hear tones and I was NOT getting answers like "the mobile number is temporary out of reach", etc.
But the iPhone of course was not ringing

Just FYI - I am based in Moscow, Russia and used MTS (local operator) SIM card.