Hey,
You have probably bought a "restricted" phone. Your provider "restricted" it to work only with a certain sim card (of theirs).
Now you cannot unlock it unless your contract is over with your provider.
If your contract (how many years you need to stay on their network) is over then you should contact your provider for an unlocking code.
Good Luck,
Tal
I own a 5210, 5140 and now..the new 3250. Symbian 60 Series OS 3.0 ..just the best.