I guess the first question would be, How do they receive email if there is no disk space? Sounds like the cluster storage needs to upgraded even if you dont use unified messaging.
For it to be true UM environemnt, the voicemail for each subsciber would need to go into thier mailbox on exchange, you could not have UM with email going to one exchange database and voicemail to another.
For voicemail only, you can add an additional Exchange server to the environment and have the voicemail stored there, but this would not be UM and still wouldnt address the preexisting cluster storage issue.
