Aastra Dialog 4420 IP Basic User Manual page 26

Outgoing Calls
26
Intrusion on a busy extension
4
You can intrude on an ongoing call on a busy extension.
Press.
Before the intrusion is executed, a warning tone is sent to the
parties in the ongoing call. A three party call is established and a
warning tone is heard.
The warning tone might be disabled for your system. The
Intrusion feature might be blocked for use on your extension
(programmed by your system administrator). If Intrusion is
not allowed, you will continue to receive a busy tone.
Dialog 4420 IP Basic