Called my cell phone and left an automated message on my voicemail: "this message is for ***** ***** a complaint has been made against you. Please call 800-whatever and reference number blah blah blah or further action will be taken." I love those empty threats. Even if the debt is real the most they can do is sue. But since I have nothing to take,  bring it. But these kinda of threats are empty because most debts aren't worth the court costs or hassle that often doesn't get the desired results and so they use it as a threat but unless you owe tens of thousands of dollars or more, they eventually will right off the debt or pass it on to another debt collector.