@Williamsays The easiest way is to try another sim card but the chances are high of it being locked if purchased more than around 3 months ago.
How to unlock is here if it is https://community.o2.co.uk/t5/How-to-Guides/Unlocking-an-O2-phone-to-use-a-different-SIM-card/ba-p/1...
All you data is backed up to Google, not your sim, so you can swap sims and sign in to new phone with your Google account and your data will transfer over automatically.
@Martin-O2 can you fix the link from the index page please?
Edit: Actually the link is fine but when pasted into a post the forum adds additional characters and screws it up.
Changes to this
I've just tried it from the index and mine works ok? (On PC using FF)
i recently upgraded and switched to the p20 pro and sold my s7 to a friend on EE. I just put in the IMEI number on my 'MyO2' account and had an unlock code within 24hrs Didn't cost anything either
Without doubt the best way, as described in the help link posted
Track usage, upgrade and get support with the My O2 appDownload My O2