IMEI blocked on iPhone XR by PTA for “Grey Traffic” without suspicious calls

If the carriers have blacklisted your phone, neither we nor Apple can help you.

Apple has no control over carrier blacklisting of phones. That is something that the carriers do, usually in response to a report by a phone's owner of a lost or stolen phone. Only the carrier can remove the phone from the blacklist – and they're not going to do that unless they have convincing proof that it is back in the hands of the owner.

If this is a phone that you purchased second-hand, you may want to read

The All Too Common SAD Reality of Buying a Used iPad/iPhone – Apple Community

Did you buy the phone used? This could be a case of buying a stolen phone and the original owner reporting it as stolen to the carrier, who blacklisted it. If you did buy a used iPhone which was reported as stolen, you likely now own a brick you can't use. And just to be clear, Apple did not, nor can they block your phone from connecting with a carrier for cellular service.

The All Too Common SAD Reality of Buying … - Apple Community

an IMEI number is a unique hardware set number in the cellular system of your phone

if carrier set their system to not perform service for devices with that number no one can change that

if you want to know more International Mobile Equipment Identity - Wikipedia