Rules on Civil Procedure
Section 8. Substituted service. - If service of pleadings, motions, notices, resolutions, orders and other papers cannot be made under the two preceding sections, the office and place of residence of the party or his counsel being unknown, service may be made by delivering the copy to the clerk of court, with proof of failure of both personal service and service by mail. The service is complete at the time of such delivery. (6a)