Iphone - sim not supported

[CathH]

I have been given an iphone by a friend. I am on 3mobile network but my sim is not supported in the iphone. I have googled but not too keen on doing the "jailbreak" downloads suggested.
Has anyone got any ideas on how I can get my sim to work?
Thanks in advance

[sherrardk]

We unlocked two iPhones - one died and is a complete goner and the other one works (sort of), but can be very tempremental.