Telephony and Telegraphy. Of a telephone or line: that is permanently for the exclusive use of the subscriber, or not connected to the public…

Originally U.S. spec. Of a telephone or telephone line: = engaged, adj. 3; (of a signal, etc.) indicating this. See also busy tone n. at compounds 2.

Telephony. Of a number or line: in use and therefore unavailable to a second caller; of a person: telephoning. Hence engaged signal, engaged-test, en…

Of encryption, or telecommunications or computer systems: protected from unauthorized access; free from the risk of being intercepted, decoded…

In the senses of the verb.