Orders for duty may be oral or written. Officers and enlisted persons may be warned for duty as follows: Either by stating the substance of the order, or by reading the order to the person warned, or by delivering a copy of such order to such person, or by leaving a copy of such order at his or her last known place of abode or business, with some person of suitable age and discretion, or by sending a copy of such order or notice containing the substance thereof, to such person by mail, directed to his or her last known place of abode or business. Orders may be transmitted by telegraph or telephone. Such warning may be given by any officer or authorized enlisted person. The officer or enlisted person 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.