Used iPhone 12 purchased locked to owner, how do I access?

It sounds like you are talking about two issues:

1. The phone is carrier-locked. Only the carrier to whom the phone is locked can release the lock. If this was the only problem, you could contact them and try to get them to release the lock. But from what I've read in other replies, carriers typically won't unlock a phone for you unless you are already their customer.
2. The phone is blacklisted. If the phone was reported as lost or stolen, carriers may have put it onto a blacklist. Absent very convincing proof that you are the original owner of the phone, carriers will not remove the phone from the blacklist, and the phone will NEVER get cellular service from those carriers ever again.

If the phone is blacklisted, there is NOTHING that we or Apple can do for you. If you can't return the phone to the customer who sold it to you, you are stuck with a "brick" that will remain on the blacklist. If this person is nowhere to be found, that may be an indication that this person knew that they were in possession of a stolen phone – and that you got scammed.

There is nothing you can do.

Apple doesn’t maintain or report devices to a blacklist.

The carriers maintain any blacklist.

The phone was reported as stolen, the person who sold it to you didn’t finish paying for it or they skipped out on their bill.

I just noticed the "locked to owner" in your title.

If the phone is both Activation Locked and on a carrier blacklist, that is an almost certain indication that the phone did not belong to the person who sold it to you, that it will be a "brick" forever, and that you got scammed.

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