unlocking old phone to sell it

[frankiepuss]

I need to sell Samsung S8 from old contract - how do I ensure it is 'unlocked'. Chat tells me to use O2 app which I don't have and cannot download as phone is offline.

[jonsie]

How to unlock here if you have already tried a different sim @frankiepuss 

Guide: Unlocking an O2 phone to use a different SIM card