One way to stop harassing phone calls from a collection agency is to send them a Cease and Desist Letter, requesting that they stop contacting you. Below is a sample Cease and Desist Letter.
Re: Account Number
I am requesting that you cease and desist with your efforts to collect on the debt referenced above. I wish to deal with the original creditor and not a collection agency.
Therefore, I request that you cease collection efforts immediately or face legal action under State and Federal consumer protection laws. I hope you would consider giving this letter the attention it deserves.