As soon as practicable after the filing of the petition, the State Corporation Commission shall set a date for the hearing. At least fifteen days’ notice of such hearing shall be given by the Commission to the petitioner and to every party in interest, but such notice to the petitioner shall be sufficient in the form of an attested copy of the order fixing such hearing sent by ordinary mail, to the address furnished by the petitioner, or to the attorney of the petitioner, if any, provided it is certified in the record, by the clerk of the Commission, that such notice was deposited in the mail at least fifteen days prior to the date set for the hearing. Otherwise such notice may be served by an officer or may be served by registered mail, return receipt requested, and, except in the case of the petitioner shall be accompanied by a copy of the petition. The petitioner shall furnish the requisite number of copies of the petition for the giving of such notices herein required.