Unlatching from O2 to move to new provider

[EmmaP27]

I moved my son from O2 to Sky mobile & got him a new phone. He wanted to keep his existing number & this has now been set up.

I want to give his old phone to my other son who will also be on sky mobile so need to unlock the phone from O2.

I have filled out the online request form but it asks for the mobile number which is now not on O2, I get an email back saying I haven’t passed the security check but I don’t know what info is wrong. I’m going round in circles, O2 won’t talk to me on the phone as it’s not urgent during lockdown.

How do I find out which question is wrong, is it the mobile number being assigned to another network now, the IMEI number or the password on the account?

[MI5]

@EmmaP27 

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

You need the number as used on O2 but the common mistake is the answer to the security question which is not your O2 password.

You can reset your security question in your MyO2.