AT&T SYSTEM 25 Manual page 166

This feature applies to Switched Loop Attendant Consoles only.
Attendant Source and Destination allows the SLAC attendant, while extending a call,
to switch back and forth between the calling party (the source) and the called party
(the destination) before connecting them together.
For more information, see the entry for "Extending Calls (Announced)" on page 3-17.
Source and Destination
Source and Destination
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