How can I unlock my SIM locked iPhone 15 Pro?

You will have to contact the carrier it is locked to. There is nothing Apple can do to help.

If you are not the customer of the cell provider that has the device locked, then they are not likely to offer any help at all as that would require personal information about the contract that the original owner had signed to lock the device to their service. You won't be able to know the terms of that agreement or if they still owe money to that provider.

If your iPhone is SIM-Locked and you are trying to get it unlocked, you need to contact your carrier, they are the only ones that can do so. Apple cannot.

But in the case that you purchased this iPhone used and found out it’s SIM-Locked, you probably won’t be able to have it unlocked by their previous carrier. In most cases carriers require that the original owner to request the unlock, and even that requires paying off the remaining balance or contract.