ACS ACS sends threatening email and hangs up on me when I call them!!! Internet

ACS sent me a threatening email just like I have seen on your site as follows:

Acount Number.........

Loan debt information

you are responsible for paying back this money and contacting us within 48m hours or we will seek legal prosecution in the court house. 

I called and questioned them as to when the last payment was made, what company it was for, and they could not give me any information. They just kept saying that they were a mother company that handles over 3000 accounts nationwide.  I told them I will check my bank account to see if this was true and they said you have this balance now are you going to pay it or not.  The more I questioned them the more agitated they became and then she hung up on me.