There is a simple solution here to stop the calls, or to get paid for the hassle.
Under the provisions of FEDERAL LAW you have the right to send a CEASE COMMUNICATIONS request to any THIRD PARTY debt collector. The FDCPA explains those rights.
Just send a CEASE COMMUNICATIONS request to these punks via CERTIFIED MAIL / RETURN RECIEPT REQUESTED. Be sure to put the certified# somewhere on the letter itself, and keep a copy of that letter for your records. Doing it this way, you have proof of exactly what you sent.
Now, wait for the little green card to come back to you. After they sign this card, and you can prove that they got your LEGAL REQUEST to CEASE COMMUNICATIONS, they cannot contact you anymore except to verify reciept of said letter, or to serve legal process on you.
If they violate the law by contacting you further, you can sue them for up to $1000 in statutory damages, plus costs.
It makes no difference if the debt is yours or not.
Make them pay!