We run VIP2000.
It's the best voicemail-system I've seen.
It handels Voicemail, FAX, ACD, Voice interception and much more. It's runs on Linux and is very stable.
We have 2 systems running. One with 8k users and one with 5k users over a net-group. It also has a builtin web-interface for all users, where they can optimize their telephony.