Whenever any schedule stating an individual or joint rate, classification, contract, practice, or rule, not increasing or resulting in an increase in any rate, is filed with the commission, it may, either upon complaint or upon its own initiative, at once and if it so orders without answer or other formal pleadings by the interested public utility or utilities, but upon reasonable notice, enter upon a hearing concerning the propriety of the rate, classification, contract, practice, or rule. Pending the hearing and the decision thereon the rate, classification,
contract, practice, or rule shall not go into effect. Except as provided in Section 455.1, the period of suspension of the rate, classification, contract, practice, or rule shall not extend beyond 120 days beyond the time when it would otherwise go into effect unless the commission extends the period of suspension for a further period not exceeding six months. On the hearing the commission shall establish the rates, classifications, contracts, practices, or rules proposed, in whole or in part, or others in lieu thereof, which it finds to be just and reasonable.
All rates, classifications, contracts, practices, or rules not so suspended shall become effective on the expiration of 30 days from the time of filing thereof with the commission or a lesser time as the commission may grant, subject to the power of the commission, after a hearing had on its own motion or upon complaint, to alter or modify them.