From:
http://www.cisco.com/univercd/cc/td/doc/product/voice/c_unity/design/udg/network.pdf
There are two types of failover available with Cisco Unity. Both types require access to the same messaging systems and messaging infrastructure components. There is local failover, in which both the primary and secondary failover servers are co-located in the same physical site as the messaging system they service. There is also remote failover, where the primary and secondary failover servers are located in separate physical sites. In a remote failover configuration, the network connectivity should be no less than 45 Mbps between the sites, and the messaging systems and messaging infrastructure components must be accessible by both Cisco Unity servers. (Note that messaging infrastructure components include domain controllers and/or directory servers, global catalog servers, and name resolution hosts.) Regardless of your network connectivity bandwidth, the response time between the Cisco Unity server and the Exchange servers it is connected to should be no more than a 40-millisecond round trip delay in order for Cisco Unity to service subscriber TUI requests normally.
Note that the requirement is 45 Mbps. This is a pretty beefy WAN link. If you don't have a 45 Mbps link you can't do failover to the other site. Also remember that if the WAN link goes down that the failover server isn’t going to have access to the Exchange server which means that subscriber won’t be able to check messages expect those left during the outage.
Thanks,
Keith
