RR: Comments elsewhere on 800Notes continue to confirm this is debt collection agency Pentagroup Financial, and not ''Sallie Mae'' or any other original creditor. THEY are the liars here. That and the rapid call pattern and the various abusive remarks from that rep give you a solid cause of action to sue PF for FDCPA violations and possibly breaking your state law.
Whether you are called a debtor or some collector thinks you can flush one out, you have rights and options. Many are simple to exercise if you only learn how. You are granted control of how you are contacted, the right to dispute debt claims, and the means to sue lawbreakers at low cost. The FTC and the CFPB can start you on your homework. See also if your state laws grant additional or stronger rights.
official FTC guidance and staff opinions on the FDCPA, PDF copy of the Act
CFPB reiteration of the Fair Debt Collection Practices Act