All hearings, and investigations, held or made by the commission, shall be public; and before proceeding to hold any such hearing or make any such investigation, the commission shall give the utility and the complainant at least twenty days’ notice of the time and place when and where such matters will be considered and determined, and all parties shall be entitled to be heard, through themselves or their counsel, and shall have process to enforce the attendance of witnesses. At the hearing held pursuant to such notice, the commission may take such testimony as may be offered or as it may desire.