Could I use the iPhone?

My mum is getting an o2 upgrade on her phone. She already has a samsung galaxy so doesn't want a new one, she said she would get a blackberry though and let me use it with my pay as you go sim. It says it's free on all tariffs, but the iPhone 3GS is also free on all tariffs. Would I be able to get this as the upgrade and put a pay as you go sim (that gets unlimited Internet) in it? Im on vocative at the moment but I also still have the o2 sim I recently switched from