This is not expected behavior. Make sure your network switch is current on firmware. We have recently found a 3rd party switch was causing phones to lose registrations intermittently and cause the phones to reset making it appear like they are rebooting. System logs may also give some idea of what is happening (Status->Support Tools -> Logs).
We have also seen a bug where if the WAN IP address changes while on a call the system reset.
Configure Multicast Paging on the Cisco IP Phone 7800 Series or 8800 Series Multiplatform Phone
The Cisco IP Phone 7800 and 8800 Series Multiplatform Phones provide voice communication over an Internet Protocol (IP) network...
Add Call Park on a Cisco 7800 or 8800 Series Multiplatform Phone Key Expansion Module
Call park allows the user of the phone to put an incoming call on hold so that the call can be retrieved on another phone. A call is park...