how to find what carrier my iPhone 4 is with (if stuck at the welcome screen), and can I activate a it with a borrowed SIM card.

OK. My friend had bought an iPhone 4 online, second hand. He does not have the SIM card. He had recently changed his passcode and FORGOT IT, and we had to restore it. The problem is that, after we restored, it askes for the SIM activation. And none of us have a MICRO SIM Card. So my question is how can I find out what carrier the iPhone is with if it is stuck on the welcome screen and if I can borrow someone elses SIM card to activate the phone so he can use it.

(we could not contact the person he bought the iPhone from to find out what carrier it is with.)