Trying to put a Sky Mobile Sim into an old unused iPhone.

[Cylina]

Basically I have my daughter's old iPhone 5 which I upgraded earlier this year. My ex husband lost his iPhone recently so I gave him the old iPhone 5 to use however, wheneve he puts his Sky Mobile Sim into the phone it keeps saying the SIM is not supported. I have been using the old iPhone 5 for work recently using a pay as you go SIM with GiffGaff and it worked fine however, it is not accepting this contract SIM.

I have tried into My Account and unlocking the phone however, I received an email from O2 stating

The IMEI has not been used on our network with the MPN
provided, so it will not be possible to unlatch it.

I need this phone to be unlocked so that my ex husband can get the use of it.

[MI5]

@Cylina 

You'll need to put an O2 sim in the phone and use it for a couple of days, then request the unlock again Guide: Unlocking an O2 phone to use a different SIM card