Thanks to all for your comments, many of which I used when I spoke to the dentist herself. I would like to stay with the dentist, if possible. We had an incompetent dentist for many years and the new one is good. As far as I can tell, there is but one other good dentist in the area. It is a field with a lot of incompetence, I'm afraid, and little recourse for patients.
The receptionist (who took payment AFTER--unbeknownst to us--sending bill to collection agency, which is perhaps illegal and definitely unethical) insists that the bill never got to the agency. I called the agency and they have no record with Mr FS's name. However, they did say to keep checking back. How long should I do this for?
I had a long chat with the dentist. She kept repeating two things: "I think $200 is a lot of money" and "I'm sorry you feel that way."
As for the first, I refrained from responding with the obvious riposte: "If it's so much money, why did you send it to collection, which takes half the money??"
As for the second: this one burns me up. There are times when it IS appropriate. But it has become a customer service cliche that--like the passive voice--absolves the speaker of responsibility. So every time she said that, I said "Are you saying that my feelings are without cause or justification?"
Finally, she did relent on this and said that she would make a personal call to patients before sending a bill to collections. I don't know how much her feelings are shaped by the $5000 in upcoming treatments we have mapped out.
I do have a lot of hostility remaining. A decent dentist a short walk down the street...that may be worth dealing with my anger.
My last (I hope!) question: how many more times should I check with the credit agency? Should I ask for something in writing from the agency and from the dentist's office before scheduling any work?
And once more: THANK YOU.