Unlock old from O2

[Breege00]

Hi. I have an old iPhone 6S which I’ve given to my sister who lives abroad. She cannot get it to work as it needs to be unlocked from O2. Can someone advise what I /she need to do please? Thanks

[Bambino · Accepted solution]

@Breege00 There's this guide, but I'm not sure it will work from abroad. Your sister may have to send the phone back to you: 

Unlocking an O2 phone to use a different SIM card. - o2uk