DAA: You'll be waiting the rest of your life for mystery call site entries to inspire any lawsuit. Complaints to attorneys general and the FTC will get a little more traction. But understand that you must confront collection agencies directly if your goal is to halt or punish their mistreatment of your privacy and consumer rights. In your case, send a cease-comm via USPS Certified with card. The FTC and many consumer watchdogs explain how to draft one.
While a third party collector is unlikely to retain one exclusive client, don't assume the big brand name lenders have clean hands in choosing their collections henchmen. Space here is too short to list the lawbreaking agencies hired year upon year by the Bailout Banks. Don't get me started on the in-house Chase bullies either, who think that nine or twelve daily calls for a few weeks is a sane and normal procedure.
All: If you are receiving repeated, misdirected, and/or abusive calls from a third party debt collector, you may find relief in the provisions of the FDCPA, the Federal law which regulates collector behavior. Learn your rights at FTC-dot-gov. See also your state laws for additional support.