new sim wont work in 7650

Hi guys/girls,
I recently purchased an O2 sim card but my nokia 7650 wont accept it, it only seems to accept vodafone sim cards any body got any ideas?, or is this what the unlocking code means? where can i get a hold of the unlocking code and how do i put it into my phone, please help, thanx

Loads of code makes on here, just read all you can about how to make and enter the codes. (little hint UEM type 5 MCC+MNC 234-15)