iPhones that are carrier locked to the O2 UK network can be unlocked using a service that involves connecting to a computer via a USB cable and using iTunes. iTunes? – you might ask, yes that’s right, and it means that it’s an official factory unlock from Apple themselves! This also means that it’s a full permanent unlock so there’s no chance of the iPhone ever locking up again after a firmware update.
Does it void the warranty of the iPhone? – That’s a simple No, since it’s a factory unlock from the Apple servers.