Place it at the end of a call scene, straight after the ringing, then follow it with the Dun Dun Dun sting to underline the failed connection, or open with a Discord call tone so the busy signal lands as the punchline to a missed chat.
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This is a short telephone busy signal, a repeating two-tone beep that sounds when a called line is already in use, lasting just under five seconds. It feels instantly familiar and slightly frustrating, like being put on hold by a phone that will not connect, and it carries a dry, old-fashioned phone humour that makes a failed call feel deliberate rather than accidental.