Unlock Iphone 3gs

I am going to buy a iphone 3gs that is currenty locked to vodafone UK, I want to unlock this phone so I can put my o2 sim in.

Before I buy can someone tell me if I will defo be able to unlock this and how it will be done. Ive only ever done jailbreaks before.....

The easiest way is to call Vodafone.

Tell them you have a 3gs that was on a vodafone contract and you'd like it unlocking.

With Vodafone it often depends on which way the wind is blowing as to whether they'll charge you or not.

Once the unlock has been processed you just put the new sim card in and connect it to iTunes.

Last month I had to unlock a 3g from O2 so it could use a Vodafone sim card. All I had to do was fill out a form on O2's website and then it took about two days. I'm sure Vodafone has a similare set up.