It shall be the duty of the Arkansas Department of Transportation to hear all complaints made by any person, firm, or corporation against any such tariff of charges so approved, to hear the parties to the controversy in person or by attorney, or both. The department may take testimony, orally or in writing, and regulate argument thereon and conduct the investigation of such complaints in such manner as to the department may seem best adapted to arrive at the truth. When any changes are made in any tariff of charges, notice thereof shall be given to the person or corporation to be affected thereby.