O2 is a Global Systems for Mobile Communications (GSM) cellular service provider that operates in the United Kingdom (UK). O2 generally sells its mobile phones with a carrier lock. This lock prevents using the mobile phone with a non-O2 Subscriber Identity Module (SIM) card. It is, however, possible to unlock an O2 phone by entering the unlock (NCK) number. Once unlocked, you can use your O2 phone with another cellular service provider's SIM card on any GSM cellular network worldwide.
Remove the back cover and battery from your O2 mobile phone.
Find the sticker with the phone's International Mobile Equipment Identity (IMEI) number. The sticker is generally located under the battery compartment. The IMEI is a 14-digit number that allows cellular service providers to identify cellular devices on their networks. Write down the IMEI number.
Call O2 customer service at 011-44-0844-809-0202 and request the unlock (NCK) number for your mobile phone. The customer service representative will ask you for the phone's IMEI number. O2 will generally give you the unlock code free of charge if you have completed the service contract for the mobile phone or if you have had an active prepaid account for 12 months or longer. Write down the unlock number.
Alternatively, you can purchase the unlock (NCK) number from a company that sells unlock codes.