Assume we can crash the RADIUS server in a VOIP system. What would be the impact on the confidentiality, integrity, and availability of VOIP service. What would be the impact if the same question if an attack occurred at a web server and a DHCP server?
This would depend on the how this is implemented (fail open versus fail closed).
But since RADIUS is used as an authentication service, I would say confidentiality and integrity wouldn't be affected (unless you have access to the RADIUS server when crashing the machine). The availability will obviously go down.
Just "crashing" the machines will not necessarily provide you access to the data in those services. If you do get access (root exploit using a buffer overflow that causes to crash the server) then you will indeed void confidentiality and integrity.