Apple unlock

My phone came out of Contract 6 months agao.

I contacted my provider EE to request an unlock.

However they said they could not unlock the phone as about 4 months into the contract the phone was replaced by Apple due to a technical fault - so the IMEI number they had was not on file and therefore not locked with them.

How do I request an Apple unlock please?

I have a new SIM ready to go but the error message says that the activation server does not recognise the Sim card/carrier.