Upon the filing of a complaint, the commission shall cause a copy thereof to be served upon the corporation or person complained of. Service in all hearings, investigations, and proceedings pending before the commission may be made upon any person upon whom a summons may be served in accordance with the provisions of the Code of Civil Procedure, and may be made personally or by mailing in a sealed envelope, registered, with postage prepaid. The commission shall fix the time when and place where a hearing will be had upon the complaint and shall serve notice thereof, not less than 10 days before the time set for such hearing, unless the commission finds that public necessity requires that such hearing be held at an earlier date.