What does the signal 10-21 mean?

The signal 10-21 means "Call by telephone." This code is used within law enforcement communications to quickly convey to officers that they need to contact an individual or department via telephone. This shorthand is part of a standardized system of codes that allows for efficient and clear communication in the field, particularly during high-pressure situations. Understanding these codes is crucial for effective coordination and response among officers.

In this context, the other options refer to different meanings that do not accurately capture the definition of 10-21. While they may pertain to various procedures or actions that can occur in police work, they do not relate to the specific instruction to call by telephone, which is vital for maintaining communication continuity.