It is not in the Wisconsin Consumer Act, I believe. It is under Money and Interest Rates Section 138.052(7s)(b). This only applies to Real Estate loans but most banks applied the same standard across the board to make it easier on their data processing system.
(7s) A person who receives loan or escrow payments on behalf of itself or another person shall do all of the following:
(a) Respond to a borrower's inquiry within 15 days after receiving the inquiry.
(b) Consider that a loan payment by check, or other negotiable or transferable instrument, is made on the DATE on which the check or instrument is PHYSICALLY RECEIVED, except that the person may charge back an uncollected loan payment.
Regulations make more sense when you are not sober.