Within 30 days after the expiration of the time allowed for filing objections to claims, the register or clerk must appoint a time, not less than 20 nor more than 60 days thereafter, for the hearing of such objections as have been filed; and of the nature of the objections and the time appointed for such hearing, he must give notice to each person whose claim is contested, by citation to be served upon him, his agent or attorney, or, if he be a nonresident and have no resident agent or attorney, such citation must be sent by mail, postage prepaid, to such claimant at his post office address.