Hospital Bills

I have some hospital bills from a surgery that have gone to collections. A collection agency called my work and I returned their call. I told the representative that I did not want them to call me at work. I then asked to provide proof of the debt and he wanted to know what that would look like to me. I told him that I wanted to know who was the original creditor and what was it for. He would not agree to send me a verification of debt. I then asked for their address so that I could communicate by mail and in case I wanted to submit a Cease and Desist letter. He again refused to give the address and stated that they know how to play hard ball with people like me. All I wanted to do was to get verification of a valid debt. What should I do? I feel that I am not being able to exercise my rights but no idea on how to ge them enforced.