The requirement that carriers file tariffs arises from §203 of the Communications Act of 1934. Carriers are then prohibited from charging or collecting any amount that differs from the tariffed rates. Carriers are then prohibited from charging or collecting any amount that differs from the tariffed rates. Parties challenging a tariff must first petition to investigate, suspend or reject a new or revised tariff filing. American Telephone & Telegraph Company, the world's largest utility, derives its interstate revenue from several classes of communication service; 80 percent is from message toll telephone service (MTS), in which the user dials his call or is assisted by an operator and pays for the service on a per-call basis. Petitioner in 73-2051, Microwave Communications, Inc. does not challenge the validity of the Commission's rate-of-return determination in Phase I of Docket 19129. AT&T has offered two service categories from the beginning of the transmission service in 1948.