Which phrase signals that urgent information needs to be communicated immediately?

The phrase "Busy unless urgent" is indicative of situations where immediate communication is necessary. This phrase suggests that while the party is currently engaged with other matters, they are open to interruptions if the information being conveyed is urgent. It sets a clear priority for communication, indicating that any urgent matters should be transmitted without delay, regardless of other activities that might be taking place.

Other phrases might imply the need for action or attention but do not specifically denote the urgency required for immediate communication. For instance, "Relay" generally indicates a need to pass on information but does not inherently convey urgency. "Fight in progress" could signify a critical situation, but it doesn't necessarily prompt immediate communication in the same way. "Repeat last transmission" pertains to clarification or reconfirming previously transmitted information, rather than indicating an urgent need for new communication.

Overall, "Busy unless urgent" clearly communicates the expectation that urgent information should override other conditions, making it the best choice for signaling the need for immediate communication.