Phone is locked

[el_as]

I have an old iPhone that I have been using with 02. Am trying to move to a new provider but when I insert the new SIM it says:

"the sim you currently have installed in this iphone is from a network provider that is not supported under the activation policy currently assigned..."

I've read online that this can mean the phone is locked to a provider. I have spoken to O2 they say the phone isn't locked to them.

when I go into phone settings it says 'Network Provider Lock' - SIM locked

Is there anything I can do to get my new SIM card working?

[Enlli]

You need to ask O2 to have the phone unlocked

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

Ways to contact O2, social media is best

Guide: How to find help & contact O2 

[MI5]

@el_as 

It could be locked by Tesco or giffgaff.

An O2 sim will work in either.

Check those providers if O2 say they haven't locked it.