Question and Confused

I am paying a medical bill off with doctor. I made the deal with the doctor's office. They will take $25. a month and use my credit card which will be available to use every month for $25.00. She asked if the debt collector had called me. I said, "no." She said, "no problem if and when they call just tell them to call the doctor's office." They called and I told them to call her. The collector was a bit put off and insisted in finding out how much I was paying them per month. I refused to inform him of that information and told him again to call her. $25. was taken from my account. Today I got another call from the collector but did not answer. What should I say if he calls again?