Idaho Code § 61-313
Except as in this act otherwise provided, no public utility shall charge, demand, collect or receive a greater or lesser or different compensation from any product or commodity furnished or to be furnished or for any service rendered or to be rendered than the rates, tolls, rentals and charges applicable to such product or commodity or service as specified in its schedules on file and in effect at the time, nor shall any such public utility refund or remit, directly or indirectly, in any manner or by any device, any portion of the rates, tolls, rentals and charges so specified nor extend to any corporation or person any form of contract or agreement or any rule or regulation of any facility or privilege except such as are specified in such schedules and as are regularly and uniformly extended to all corporations and persons: provided, that messages by telephone or cable, subject to the provisions of this act, may be classified by the utility into day, night, repeated, unrepeated, letter, commercial, press, government and such other classes of messages: provided further, that nothing in this chapter shall be construed to prevent telephone and cable companies from entering into contract with common carriers for the exchange of service at rates common to all common carriers of like class.
Idaho Code § 61-313