Change sim from O2 to different provider

[ej5]

I have a very old phone and want to use it with a different Sim (from Sky) in addition to my current phone with O2. My phone number seems to be attached to the very old one as well as the current phone, and I am not sure how to change sim to a sky sim in the old phone without upsetting my current phone 

[ej5]

I need O2 to 'unlock' the old phone 

[Enlli · Accepted solution]

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

[MI5]

@ej5 

Have you tried the sky sim in the phone as it may work without unlocking?