When you upgrade your system to this software, the new feature distinctive ring will be enabled. Your existing DECT handsets will have the old menu structure where only Internal and external ringtones can be selected. To get the new DECT handset menu structure you need Re-Register the DECT handsets, there is no need to DE-register the handset first. How this must be done you can read on the distinctive ring page.
Or you can disable the distinctive ring functionality.