@COnnar It won't make any difference to your phone https://community.o2.co.uk/t5/How-to-Guides/Unlocking-an-O2-phone-to-use-a-different-SIM-card/ba-p/1...
Unlocking wouldn't affect anything other than the phone you choose to unlock. Guide here
Best of luck and welcome to the forum
*The Game Is On*
@COnnar you need to give the IMEI number of the phone you want to unlock.
@COnnar Just follow the guide I posted above.
Track usage, upgrade and get support with the My O2 appDownload My O2