Check cashed and collection

What is the name of your state? AZ
I have a question about a debt that I paid to a dentist.
I was informed by their office in August 2002 that there was a dispute with the insurance company about a bill, but was assured that all would be taken care of and would be "written off".
Well, in February 2003, I receive a bill from the dentist office stating that I owed them $200.00 and expected payment immediately. I called and spoke with the new office manager, "Christine". She explained they made a mistake and that was my problem. I attempted five times to reach the dentist by phone, but she wouldn't give him the messages. So, I sent a registered letter to him along with an explanation of events that transpired and a check in the amount of $100.00 with "Paid In Full" written in the memo section. Meanwhile, Christine sent this to collections. That agency has called about five times per day.
The one time that I spoke with the collections guy, he stated that the "paid in full" comment means nothing unless it is written on the back. Is this true? I cannot seem to find an answer anywhere. Any help would be greatly appreciated! Thanks.

No, it's supposed to be written in the memo field.

Here's a link:
[url]http://www.azleg.state.az.us/search/oop/qfullhit.asp?CiWebHitsFile=/ars/47/03311.htm&CiRestriction=%22accord%20and%20satisfaction%22&CiQueryFile=/search/ars.idq&CiBeginHilite=[/url]<b>&CiEndHilite=</b>&CiUserParam3=*****/ars/title47.htm&CiHiliteType=Full

Now, send these jerks a Cease and Desist, citing Arizona Revised Statute 47-3311. You're done.

Thanks Kevinss. I appreciate this!
This should have them with their tale between their legs.