Okla. Admin. Code § 165:55-9-3
Unless otherwise authorized by the Commission, bills shall be payable immediately upon receipt and past due fifteen (15) days after the date of the telecommunications service provider mailing or after any deferred payment date previously established either by oral or written agreement between an end-user and the telecommunications service provider. The date after which the bill is past due shall be stated on the bill. If the bill is not paid when past due, the telecommunications service provider may apply late payment charges of 1.5% unless otherwise approved on any unpaid balance for regulated telecommunications services as provided in its filed, approved tariffs. Payment shall not be considered late if it is received by the due date at an authorized office.
Okla. Admin. Code § 165:55-9-3