NEC DT900 Series - There Is no Audio to My Headset; Audio Quality

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AUDIO
DT900/DT900S Series Phone User Guide 11-167
There is no audio to my headset
1 Verify that the headset is firmly plugged into the HEADSET jack on the back of the
telephone.
2 Make sure the handset for your telephone is on-hook.
3 Make sure the headset is enabled. You should see the Headset icon on the LCD when
you dial or answer a call if the headset is enabled.
4 If you do not see a Headset softkey, verify that the Headset is enabled for your phone:
a. Press the Menu key.
b. Select 3 User Settings.
c. Select 3 Headset
d. Select 1 Enable/Disable.
e. Select 2 Enable and press the OK softkey or b Enter key on the cursor pad.
5 Try the telephone handset to verify if the problem is with the headset. If you can hear
audio through the telephone handset, try another known-working headset with the
phone.
6 If you are still having problems with your phone, contact your system administrator.
Audio quality
If you are having issues with the audio quality on your phone, there may be a problem
with your connection to the network. Contact your system administrator.