Section 20. Whenever the department receives notice of any changes proposed to be made in any schedule filed by any common carrier not furnishing the service of transmission of intelligence by electricity under this chapter, it may, either upon complaint or upon its own motion, and after notice, hold a public hearing and make investigation as to the propriety of such proposed changes. Whenever the department receives notice of any changes proposed to be made in any schedule filed under this chapter which represent a general increase in rates by a common carrier furnishing the service of transmission of intelligence by electricity, it shall notify the attorney general of the same forthwith, and shall thereafter hold a public hearing and make an investigation as to the propriety of such proposed changes after first causing notice of the time, place and the subject matter of such hearing to be published at least twenty-one days before such hearing in such local newspapers as the department may select. Pending any such investigation and the decision thereon, the department may, by order served upon the common carrier affected, suspend, from time to time, the taking effect of such changes, but not for a longer period than ten months in the aggregate beyond the time when the same would otherwise take effect. After such hearing and investigation, the department may make, in reference to any new rate, joint rate, fare, telephone rental, toll, classification, charge, rule, regulation or form of contract or agreement proposed, such order as would be proper in a proceeding under section fourteen. At any such hearing involving any proposed increase in any rate, joint rate, fare, telephone rental, toll or charge, the burden of proof to show that such increase is necessary to obtain a reasonable compensation for the service rendered shall be upon the common carrier. If, at a hearing involving any proposed decrease in any rate, joint rate, fare, telephone rental, toll or charge demanded by any common carrier, it shall appear to the department that the said rate, joint rate, fare, telephone rental, toll or charge is insufficient to yield reasonable compensation for the service rendered, the department may determine what will be a just and reasonable minimum to be charged, and make an order that the common carrier shall not thereafter demand or collect less than the minimum so prescribed without first obtaining the consent of the department, after a public hearing.