It can't just be unlocked free with a code, you need to find somewhere with a DM3 box like fonefunshop in sheffield.
If it's a payg phone you can't get the unlock code until the phone is a year old, if it's contract you can't until it's expired..
Other alternative is a turbo sim from ebay, that will make the phone think it's unlocked.
GATORS
May 2009