The compat settings are located on the left side of the base.
They are only of use when in LIFTER MODE.
A - G settings.
D is the most common setting. This is how it comes set out of the box.
As with any headset, if D is not working, test all the different compat settings.
PLEASE NOTE - Cisco phones should be on compat A in the headset jack and D in the handset jack.