Vodafone is one of the largest mobile phone providers in the United Kingdom. O2 network is one of Vodafone's main rivals. If you intend to switch from Vodafone to O2, you'll need to unlock the phone to get the SIM (subscriber identity module) card to work. This simple process can be completed in a few minutes if you have the right codes.
Turn your Vodafone mobile phone on and dial #*06# on the keypad. Write down the 15- to 17-digit IMEI (international mobile equipment identity) number that appears. You will need this to create the unlock code.
Choose an online store that sells unlock codes (see Resources). As of 2010, unlock codes cost approximately £13 apiece.
Go to the unlock site and enter the IMEI code. Select Vodafone as your network and the United Kingdom as your country. Enter your e-mail address and your payment information. You should receive your unlock code by e-mail in a few days.
Turn the Vodafone mobile off and take out the battery. Take out the phone's memory chip (SIM card), the white plastic chip underneath the battery. Insert your new O2 SIM card into the phone, put back the battery and turn it back on.
Type in the unlock code you purchased. Click "OK" and wait for the phone to restart. Once the phone comes back on you will see the O2 network logo at the top of your screen.