Wis. Admin. Code Department of Financial Institutions DFI-WCA 1.36
Should a customer or an authorized representative question in writing any bill or statement of a merchant, or of an assignee where notice of assignment pursuant to s. 422.409, Stats., has been given, such merchant or assignee shall in accordance with s. 422.306, Stats., respond specifically to the issue or dispute raised by the customer within 30 days of receipt of such inquiry, or, in the case of transactions evidenced by open-end credit plans not later than 2 complete billing cycles (in no event more than 90 days) from receipt of such inquiry. Inquiries made on an instrument of payment of [or] the returnable portion of the billing statement need not be acknowledged if the creditor conspicuously discloses this requirement on the statement or other disclosure to customers regarding the correction of billing errors. A reasonably disputed debt under s. 427.104(1) (f), Stats., shall include an indebtedness questioned under this rule from the date of notice to the merchant to the date the merchant's response is made.
Wis. Admin. Code Department of Financial Institutions DFI-WCA 1.36