I bought a Nokia 82 someting and that was locked. Even though I bought it as PAYG. So, it depends. 02 usually have all contract phones unlocked, however, the iPhone was the exception.
Ah now you've reminded me. Why is this fair? If O2 sell all their contract phones unlocked, then how come the iPhone is locked? As I said earlier I'm on O2 at work and all of my work phones I've been able to put any SIM in. I can only use T-mobile at home it's the only signal I get.
I really do wish they would just sell an unlocked version and have done with it. Or maybe I should just persuade work that I need one