Adding 3rd party SIP device to CUCM is pretty straightforward: add End User with digest credentials, create SIP profile with Digest authentication support, add 3rd party SIP device (choose advanced for video) and apply config on the device itself (you should usually specify DN, end user name and digest credentials). Here is the link to cisco.com which describes the process.
But recently I encountered that my Lifesize video codec refused to register on CUCM. TCPdump showed that device sent SIP register, CUCM replied with 401 Unauthorized with all necessary info for authentication (Digest realm=”ccmsipline”, nonce=”some long random line”, algorithm=MD5), which was OK. When Lifesize calculated the response and sent it with subsequent SIP REGISTER, CUCM replied with 500 Internal Server Error:
Before submitting a TAC case I decided to check device config on CUCM. There is a configuration parameter of Device Pool assigned to device. This pool define CMgroup among other things. Cisco phones download their configuration files where they can find CUCM nodes they can register to, which is not the case with 3rd party SIP devices.
Now back to Lifesize configuration window: in SIP Registration hostname field I specified a node which was not in the CMgroup assigned to the device. When I changed it to appropriate hostname, Lifesize registered normally.
smirnov-am's tech blog 