Bottom line..
Do not talk to them, do not do ANYTHING. You already kinda fucked up by sending them any money. However the fact they took it and then said they don't accept your payment plan, I'd dispute the money with your bank, get it back and tell them to fuck off.
Find out who they are thats calling and their address, send them whats called a "drop dead letter" simply stating they must send you written correspondence along with documented details on the debt you actually owe.
http://clarkhoward.com/topics/drop_dead_letter.html
(Date)
To Whom It May Concern:
I have been contacted by your company about a debt you allege I owe. I am instructing you not to contact me further in connection with this debt. Under the Fair Debt Collection Practices Act, a federal law, you may not contact me further once I have notified you not to do so.
Sincerely,
(Name)
(Account No.)