If it is a collection service agency as defined under the Tennessee Collection Services Act or a debt collector as defined by the Fair Debt Collection Practices Act, you can write to them and tell them to "cease and desist" from calling you. Demand that all communication is put into writing.
With that said, it is probably a debt collector and said debt collector is probably Affiliated Creditors or Fox Collection. They do most of the collection work for medical providers in the mid-state. Be advised that medical providers in the mid-state sue for very paltry amounts.
My advice is to look at your credit reports now and let an attorney view them as well. They're are certain federal laws that apply to credit reporting which may offer you a way out of getting sued. Moreover, an attorney may be able to settle these debts for less than what is being asked.
I am not your attorney nor is any answer I may provide legal advice. You may contact me directly for legal advice only if you are a resident of Tennessee insofar as I only practice in Tennessee.