(b) Whenever there is filed with the commission any schedule stating a change in the rates or charges, or joint rates or charges, or stating a new individual or joint rate or charge or joint classification or any new individual or joint regulation or practice affecting any rate or charge, the commission may, either upon complaint or upon its own initiative without complaint, enter upon a hearing concerning the propriety of the rate, charge, classification, regulation, or practice; and, if the commission so orders, it may proceed without answer or other form of pleading by the interested parties, but upon reasonable notice, and, pending the hearing and the decisions thereon, the commission, upon filing with the schedule and delivering to the public utility affected thereby a statement in writing of its reasons for the suspension, may suspend the operation of the schedule and defer the use of the rate, charge, classification, regulation, or practice, but not for a longer period than 270 days beyond the time when the rate, charge, classification, regulation, or practice would otherwise go into effect; and after full hearing, whether completed before or after the rate, charge, classification, regulation, or practice goes into effect, the commission may make the order in reference to the rate, charge, classification, regulation, or practice as would be proper in a proceeding initiated after the rate, charge, classification, regulation, or practice had become effective: Provided, That in the case of a public utility having 2,500 customers or less and which is not a political subdivision and which is not principally owned by any other public utility corporation or public utility holding corporation, the commission may suspend the operation of the schedule and defer the use of the rate, charge, classification, regulation, or practice, but not for a longer period than 120 days beyond the time when the rate, charge, classification, regulation, or practice would otherwise go into effect; and in the case of a public utility having more than 2,500 customers, but not more than 5,000 customers, and which is not a political subdivision and which is not principally owned by any other public utility corporation or public utility holding corporation, the commission may suspend the operation of the schedule and defer the use of the rate, charge, classification, regulation, or practice, but not for a longer period than 150 days beyond the time when the rate, charge, classification, regulation, or practice would otherwise go into effect; and in the case of a public utility having more than 5,000 customers, but not more than 7,500 customers, and which is not a political subdivision and which is not principally owned by any other public utility corporation or public utility holding corporation, the commission may suspend the operation of the schedule and defer the use of the rate, charge, classification, regulation, or practice, but not for a longer period than 180 days beyond the time when the rate, charge, classification, regulation, or practice would otherwise go into effect; and after full hearing, whether completed before or after the rate, charge, classification, regulation, or practice goes into effect, the commission may make the order in reference to the rate, charge, classification, regulation, or practice as would be proper in a proceeding initiated after the rate, charge, classification, regulation, or practice had become effective: Provided, however, That, in the case of rates established or proposed that increase by less than 25 percent of the gross revenue of the regulated public service district, there shall be no suspension period in the case of rates established by a public service district pursuant to §16-13A-9 of this code and the proposed rates of public service districts shall go into effect upon the date of filing with the commission, subject to refund modification at the conclusion of the commission proceeding. In the case of rates established or proposed that increase by more than 25 percent of the gross revenue of the public service district, the district may apply for, and the commission may grant, a waiver of the suspension period and allow rates to be effective upon the date of filing with the commission. Notwithstanding the provisions of subsection (e) of this section, the public service district shall provide notice by Class I legal advertisement in a newspaper of general circulation in its service territory of the percentage increase in rates at least 14 days prior to the effective date of the increased rates. Any refund determined to be due and owing as a result of any difference between any final rates approved by the commission and the rates placed into effect subject to refund shall be refunded by the public service district as a credit against each customer's account for a period of up to six months after entry of the commission's final order. Any remaining balance which is not fully credited by credit within six months after entry of the commission's final order shall be directly refunded to the customer by check: Provided further, That if any such hearing and decision thereon is not concluded within the periods of suspension, as above stated, the rate, charge, classification, regulation, or practice shall go into effect at the end of the period not subject to refund: And provided further, That if any such rate, charge, classification, regulation, or practice goes into effect because of the failure of the commission to reach a decision, the same shall not preclude the commission from rendering a decision with respect thereto which would disapprove, reduce, or modify any such proposed rate, charge, classification, regulation, or practice, in whole or in part, but any such disapproval, reduction, or modification shall not be deemed to require a refund to the customers of the utility as to any rate, charge, classification, regulation, or practice so disapproved, reduced, or modified. The fact of any rate, charge, classification, regulation, or practice going into effect by reason of the commission's failure to act thereon does not affect the commission's power and authority to subsequently act with respect to any such application or change in any rate, charge, classification, regulation, or practice. Any rate, charge, classification, regulation, or practice which shall be approved, disapproved, modified or changed, in whole or in part, by decision of the commission shall remain in effect as so approved, disapproved, modified, or changed during the period or pendency of any subsequent hearing thereon or appeal therefrom. Orders of the commission affecting rates, charges, classifications, regulations, or practices which have gone into effect automatically at the end of the of the suspension period are prospective in effect.