If I'm not mistaken unlocking phones is illegal. Now as many of us probably do have unlocked phones (I have no cell phone) we all know the value and importance of it, but this is not the site to be discussing such matters. If you can't find it on google, try going to a local flee market or outlet/strip mall and try to find someone who runs a little phone shop, they usually unlock phones.
As far as i know unblocking or changing the IMEI/Serial number of a phone is illegal, however you can still unlock them for other networks, though i think this is only the case if you own the phone (ie: if you got it on contract and the contract hasn't expired it isn't actually your property) RwD
Go down your local Market or Boot Sale, and ask the Blokes in the mobile phone stalls. normally £15-£20.
I don't think it's illegal if you go through the correct channels, ie. the phone network provider it came from. I know and have found out that Orange charge £20.
Unlocking a mobile phone isn't illegal. In fact quite the opposite, it is your consumer right to have your handset unlocked. The networks have to provide you with an unlock code if you request it. Although they will charge approximately a much higher amount to do it than some internet/market vendors because they assume you will be not using their network. What is illegal is changing IMEI number of a mobile phone.