Use O2 iphone 3G (3.1.3) on Vodafone?

I have an iphone 3g, purchased a few years ago on O2, and i'm trying to get it to work on my Vodafone sim card (because we don't have a signal for O2 in my house - which is where i am most of the time). It has been jailbroken before but to preserve the iPhone's "originality" as it were, i decided to get rid of it and install 3.1.3. I borrowed my dad's O2 sim card from his iPhone 3GS to "register it" so i could use it for the apps music etc. but i now want to get it working on vodafone.
(I also reset it because i need to take it to a genius bar to get the headphone socket fixed)
How can i unlock it - at least until i take it in?

P.s. i'm using a MacBook not windows
 
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