The SPA942 is not supported with CME. But even though you have the SPA942's registered it still should not cause the CME router to reboot. Please open a support call on the 2811 CME router to debug the issue properly. The support team can recommend the support IP Phone endpoints.
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...