Letter of Default !

Send the law firm a letter stating that you are current on your mortgage, and asking them, if they have any proof of default, to provide. In the letter, state also that you are checking with your lender and that if it turns out, after investigation, that you do owe anything, you will pay the amount. Demand (politely but firmly) that they take no action until and unless they can prove that you owe money and, if you do, you have a chance to verify the amount and make payment if appropriate. Send this overnight with tracking, so you can verify delivery.

Simultaneously, contact your lender. Ask them to verify (1) that you are current; and (2) to explain why this law firm is contacting you. Send them a copy of the law firm letter.