Can I buy a sony ericsson w850i on Vodaphone and get a unlock code?

I want to buy my son a sony ericsson w850i for xmas, i've found a cheap one but it is on vodaphone. We don't get a good vodaphone signal here and really wanted it on O2 but the phone is 拢100 dearer. Can I contact Vodaphone and unlock this phone so I can put an O2 simcard in it for him?|||if its from carphone warehouse then all the phones are allready unlocked itb says it is vodafone but it can take all sim cards if not then buy it and go to a market and get it unlocked there for a fiver