The Fair Debt Collection Practices Act only deals with consumer debt. I think that your business loan would not qualify for protection under that act, even though it is personally guaranteed.
However, you have some other options: (1) refinance the debt, if you have credit available; (2) try to contact the original lender to discuss payment options; or (3) file bankruptcy. The third option no longer carries the negatives it once did and it would cease all collection activity immediately. You'd have to file bankruptcies for both the business and you personally, but it might be the fresh start you need and get the nasty collection agents off your back.
You should speak with an attorney to discuss your options (and if there are more I missed here, which could include suing the agency for common law harassment).
Have an attorney send them a letter stating all communications should be in writing with the attorney and they will cease and desist or they will be violating the FDCPA.