[jpd1977uk]

I currently have a BB Curve 8520 on Virgin Mobile UK.

I was wondering if I can put another SIM card in and use my PIN number on the new network, O2UK PAYG, (Yes, I know I need the B.I.S enabled) or do I have to ask Virgin to relase my PIN?

[MidnightDraven]

You would need to get Virgin Mobile to delete your BIS account.
This releases the PIN.
You'd also need to make sure your BB is unlocked to use a different network.