Changing Service providers software.

I have just aquired a secondhand 7130g and it's on Vodafone. The write up said it used to be on BT Cellnet which is now O2 but it doesn't work with my O2 sim. If I install O2 software will it work or do I have to get the phone unlocked first.

Thanks in advance, Jamie.

The phone must be unlocked, unless it was an O2 (BT Cellnet) device in the first place...

Thanks, thought that might be the case. Why is nothing easy?!