I have a remote user who cannot get a dial tone in his headset. He has confirmed the headset is correctly plugged in, charged and the lifter does activate when he presses the button. The line goes active, and he can use speaker to hear the dial tone. He did a reset, and re-sync. No luck. I sent him another 'known-good' headset - no luck. He does not have a headset button on the phone. What else can I check. I've googled this times a million. Thanks!!