How do I unlock my iphone 5 handset
I purchased an iphone 5 handset from a work colleague when she upgraded to an iphone 6. I need to unlock the handset (currently locked to O2) so that my son can use it with his 3 network sim card (this is on a contract). I've called the O2 Customer Service number but it is completely automated as is asking for a phone number which I obviously don't have. As I understand that only the carrier can unlock the handset, I appear to have hit a brick wall.
Anyone out there who can help? We are located in the Nottingham/Mansfield area.
Thanks,
iPhone 5, iOS 8.3