Orders for duty may be oral or written. Officers and enlisted men may be warned for duty either (a) by stating the substance of the order, (b) by reading the order to the person warned, (c) by delivering a copy of such order to such person, (d) by leaving a copy of such order at his last known place of abode or business with some person of suitable age and discretion or (e) by mail directed to him at his last known place of abode or business. Orders may be transmitted by telegram or telephone. Such warnings may be given by any officer or enlisted man. The officer or enlisted man giving such warning shall, when required, make a return thereof, containing the names of persons warned and the time, place and manner of warning. Such returns shall be verified on oath and shall be prima facie evidence on the trial of any person returned as a delinquent of the facts therein stated.