All answers by the garnishee shall, concurrently with filing, be served upon the plaintiff or his attorney. Service may be shown by the written acknowledgment of the plaintiff or his attorney, or by the certificate of the garnishee or his attorney, attached to the garnishee's answer, that a copy of the answer was mailed to the plaintiff or his attorney; provided, however, no service shall be required unless the name and address of the plaintiff or his attorney shall appear on the face of the summons of garnishment; provided, further, that, if the garnishee fails to serve the plaintiff, the plaintiff shall be allowed 15 days from the time the plaintiff receives actual notice of the answer to traverse the same.