(1) The commission as regards a complaint issued by the commission, or upon receipt of a formal written complaint which is in substantial compliance with these procedural rules and which appears to state a cause of action within the commission's jurisdiction, shall cause a copy thereof to be served upon each defendant, accompanied by a notice from the commission calling upon each defendant to satisfy the complaint, or to answer the same in writing, within such reasonable time as may be specified by the commission in such notice. Service in all hearings, investigations, and proceedings pending before the commission shall be made personally or by first class mail.