Restriction code for Tesco mobile on Nokia 6310i?

I am trying to understand how unlocking works. I have an 'unlocked' 6310i. It used to be on Orange. Does that mean it is unlocked from Orange but still locked to other networks???? When I put in a Tesco mobile sim it is asking for a restriction code. What's the difference between unlocked and restricted? I've looked at various websites offering unlocking but I think the phone is already unlocked. So do I just need a restriction code which will let me use the Tesco sim?