The features that you will lose are the automatic tracking of phone through your network.

CER will still know what phones are attached to the CallManager cluster, but it will not be able to use CDP to auto discover and track the devices if they move through your network. As as administrator, you will be able to manually assign and change an phone's Emergency Response Location (ERL), but if the phone moves from one location to another, the administrator must update the ERL assocation so the emergency call is placed to the right PSAP.

Thanks,

Dan Keller