If when using the PRESENCE with Microsoft Lync screen sharing the PRESENCE begins to mute and un-mute randomly that indicates there is an interaction issue occurring between the headset and Microsoft Lync software. This issue was resolved through firmware updates so ensure that the latest firmware version has been installed on the PRESENCE.
Firmware updates can be obtained through the Sennheiser Updater: (http://en-us.sennheiser.com/telecommunication-business-support-service-support-software).
To install firmware connect the PRESENCE to the computer via the USB cable. Ensure the PRESENCE is deactivated or the Sennheiser Updater will not recognize it. Once the Sennheiser Updater software has recognized the PRESENCE will appear in the Sennheiser Updater. Click on the "OPTION" tab at the top of the Sennheiser Updater and then select the "Check For Updates" option. Any new firmware updates will appear and can be installed by clicking the "UPDATE" button.