49 CFR §220.33

1. (a)Upon receiving a radio call, an employee shall promptly acknowledge the call, identifying the employee's station in accordance with the requirements of § 220.27 and stand by to receive. An employee need not attend the radio during the time that this would interfere with other immediate duties relating to the safety of railroad operations.
2. (b)An employee who receives a transmission shall repeat it to the transmitting party unless the communication:
   1. (1)Relates to yard switching operations;
   2. (2)Is a recorded message from an automatic alarm device; or
   3. (3)Is general in nature and does not contain any information, instruction or advice which could affect the safety of a railroad operation.