As mentioned by Carlosdl, SIP proxy is the one in middle of the caller and callee, it determines the callee and forwards the call and forks the incoming call to different destinations. It also combined with the resource manager in an ISP deployment scenario. Assume that the virtual machine also has the access to same CPU in its own way.
Proxy combined with the Media server utilizes most of the resources. So it is purely depending on the deployment model and the storage.
We need to think how to maintain the backup in case of one vm goes down and another will take up the role and continue to provide the service. Backup model should learn the current status of the call and continue without any disturbance.