Vodafone is offering a free service for all it's Vodafone customers who have purchased a phone through them that is locked to their network to unlock their mobile phone completely free of charge!
You need to be a Vodafone customer and your phone has to be registered in your name, this might also work on phones which have expired contracts as they did confirm it will work but my phone itself was within the contract period.
You need your phones IMEI number which you can get by typing *#06# on your phone which you want unlocked, you will then be presented with a bunch of numbers, write this down, now you need to call Vodafone customer services on 191 and request for your phone to be unlocked, make sure you explain that you are a customer and that you have a contract/had a contract and want it unlocked, they will ask you for your IMEI number, give it to them.
I've just unlocked my iPhone 4 and it works perfectly on Tesco Mobile & T-Mobile! Enjoy this offer Guys!