Do you have other DX80's registered to the same cucm which are working fine? If yes, then the issue could be specific to this DX80. You may compare the switchport config and device pool settings of this DX80 with the working ones. If this is the only DX80 in the setup and already running the latest firmware then you may need to get the debugs from the phone and corresponding application logs and callmanager traces from cucm to see what is causing the issue.
If the issue started after a firmware upgrade then you may try downgrading it back to previous firmware to see if the issue goes away and check with TAC for any known issues with this firmware.
Manish
