Due to this the way to make bridged calls with USB has changed as well.
When you have an active analogue/mobile call on the Konftel 300 just press the LINE MODE-button and select USB to bridge the USB call.
When you have an active USB call and want to add analogue/mobile to the call.
Press the green off-hook button, then press the LINE MODE-button and select USB,
(which was put on hold when you activated the analogue/mobile connection) to bridge the two connections.