Carrier lock

They mislead you, only the carrier it is locked to can unlock it, Apple cannot. Call them back and escalate the issue until you reach someone who knows what they are talking about.

How to unlock your iPhone for use with a different carrier - Apple Support

If the phone is carrier-locked, the carrier gave you bad information. Apple does not release carrier locks. Carriers do. The carrier to which the phone is locked is the entity who has the power to unlock it.

If the phone is actually Activation Locked – locked to a particular owner – then it is useless to you. Activation Lock is an anti-theft device meant to "brick" lost and stolen iPhones.

You were misinformed about contacting Apple to remove the Carrier Lock. This can only be done by the carrier that it is locked to. Carriers are not obligated to unlike a phone for a third party as the contract with the original owner may have not been met to unlike the device and getting information about the original owners contact is usually stopped with privacy concerns.

The only thing you can do is work with the carrier and ask them if there is anything you can do to unlock the phone.