PLEASE ANSWER, VERY IMPORTANT! can anyone help me find collection agency harrasment laws?

Question

i believe i am being harrased by a collection agency. they are calling up to 5 times a day, with phone calls as late as nine pm. ive already set up a payment plan with them yet they continue to call. i live in the state of wisconsin, but i cant seem to locate state specific laws on this type of situation. can anyone help me find the WISCONSIN state specific laws about being harassed by a collection agency PLEASE?

i know its not illigal for them to want the money, but i am CERTAIN there are guidelines stipulateing when and how often they can call. five times a day is a little exessive, espcially since i have a payment schedule set up, and explained this all in full to two of the people who called me