29 CFR 785.15: On duty.

A stenographer who reads a book while waiting for dictation, a messenger who works a crossword puzzle while awaiting assignments, fireman who plays checkers while waiting for alarms and a factory worker who talks to his fellow employees while waiting for machinery to be repaired are all working during their periods of inactivity. The rule also applies to employees who work away from the plant. For example, a repair man is working while he waits for his employer's customer to get the premises in readiness. The time is worktime even though the employee is allowed to leave the premises or the job site during such periods of inactivity. The periods during which these occur are unpredictable. They are usually of short duration. In either event the employee is unable to use the time effectively for his own purposes. It belongs to and is controlled by the employer. In all of these cases waiting is an integral part of the job. The employee is engaged to wait. (See: Skidmore v. Swift, 323 U.S. 134, 137 (1944); Wright v. Carrigg, 275 F. 2d 448, 14 W.H. Cases (C.A. 4, 1960); Mitchell v. Wigger, 39 Labor Cases, para. 66,278, 14 W.H. Cases 534 (D.N.M. 1960); Mitchell v. Nicholson, 179 F. Supp, 292,14 W.H. Cases 487 (W.D.N.C. 1959))