AT&T SYSTEM 25 Manual page 146

This feature applies to Switched Loop Attendant Consoles only.
While extending an incoming call, the Join feature allows the attendant to connect the calling
party, the called party, and the console in a three-way call. All parties can talk to each other.
Once Join is in use, however, there is no way to "unjoin" the calling and called parties (that
is, separate them back into SOURCE and DEST). However, the attendant can drop out,
leaving the caller and called party connected.
The attendant uses the Join feature to stay on an extended call and give assistance to the
other parties. A joined call can be expanded into a conference call by adding more parties.
The attendant activates this feature by pressing JOIN while still connected to one of the other
parties. The attendant can then drop out and leave the other parties connected by pressing
RELEASE. To drop all parties, the attendant can press FORCED RELEASE.
For more information, see the entries for "Extending Calls" on pages 3-17 and 3-21.
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