I don’t have that headset, but I do have a Sennheiser wireless headset running on a 6930.
Likely you’ve already tried this, but:
Under the “Audio” settings for the phone,
– Make sure DHSG is turned on in the setting field under “Headset”.
– Make sure you have selected one of the options that includes “Headset” under the “Audio Mode” options
Without having a headset of the same model, I can’t really offer much more help.