What code would be used to refer to a phone call in this context?

In the context of communication codes, "10-21" specifically refers to a phone call. This code is part of the Ten-Codes or Ten Signals, which are often used in law enforcement and emergency services to convey information succinctly. The clarity of using "10-21" helps ensure that the message is easily understood among personnel, allowing for efficient communication regarding making or receiving telephone calls.

The other options represent different meanings within the Ten-Code system. For instance, "10-100" generally denotes a restroom break, while "10-4" is a confirmation signal meaning "acknowledged" or "understood." "10-22" typically indicates to disregard a previous order or message. Each of these codes serves a specific purpose, but "10-21" is unique in its relation to phone communication, making it the clear and correct choice for referring to a phone call within this context.