See also:

Fluid Capacities
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Conference Call
When two calls are in progress (one active and one on hold), press and hold the button until you hear a double beep indicating that the two calls have been joined into one conference call. ...

Toggling Between Calls
If two calls are in progress (one active and one on hold), press the button until you hear a single beep, indicating that the active and hold status of the two calls have switched. Only one c ...