How is the phone and the DN configured to accept the call? If it is AutoAnswer w/ Headset then the phone will never ring and the call will just drop in on the user's headphone. If it is not set to Auto Answer then the phone should be ringing. Also, what is the DN set to for Ring Setting Phone Idle? Make sure it is set to Ring (or default if the default is Ring) and not Disable. Lastly you can test the functionality of the ringing to see if there is a change when the headset light is on vs off. i.e. if you are in idle with the headset light lit and a call comes in, does it ring the phone outloud or only through the headset? If only through the headset then turn off the headset until a call comes in (i.e. press the headset button to turn the light off).
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