Paying Off a Subject-To Mortgage and Finding Out the Remaining Balance

Who has experience with dealing with "subject-to" mortgages? If someone took over one from another person, and the mortgage company is not answering to the new owner of the house since the mortgage is still in the old name, but the new person has the deed to the house in their name, what is the recourse?

Sending a certified letter with the proof of sale / deed to the company to release the info on the remaining balance?