While it is technically possible to upgrade to Enterprise Edition we only support Standard Edition (same as voicemail run-time) installed on the Unity server. See:
http://www.cisco.com/univercd/cc/td/doc/product/ voice/c_unity/sysreq/246sysrq.pd
The main reason we have this policy is because we don't want too much load on the Unity box itself. So basically if you are at the point where you require Enterprise Edition you are exceeding the bounds of a 2.4.6 voicemail only install.
My recommendation (and the only supported option) would be break out eseutil and defrag the database. Once it has been compacted it should mount and then you want to setup storage limits to prevent this situation from happening again.
Thanks,
Keith