My father and I have had similar experiences with a company telling us that we owed money when we did not. The company did what is called a 'skip trace.' Not fun. Eventually we had to demand to talk to the CEO of the company. The calls had escalated to 3 times a day. The CEO was kind, said he would take care of it, and gave us his HOME phone number in case we had any further problems. I highly recommend this course of action.
They start calling early and continue calling sometimes till almost 10pm...at least 5 calls a day. I do so wish that this number could be blocked and that the person on the other end of the line would step out in front of a bus.