AT&T System 25 - Manual (Page 49)

Call Waiting
❑
Some telephones in your System 25 may have the Call Waiting
feature. When you dial the extension of a person who has this
feature and their line is busy, you will hear a special ringback
tone. This tone indicates that the line is busy, but the person
has been notified of a waiting call. (You can still send a call to
that extension.)
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