One of the recent issues I have come across in an OCS Enterprise Voice deployment is the ability to alternate route after a gateway becomes unavailable.
In the October updates for the mediation server, KB957490 was resolved which provided a fix for redundant gateways and alternate routing when there is a gateway failure. Previously a failed gateway would just initiated a 500 response code but with this update, a 504 message is sent when a gateway become unreachable. This forces OCS to look for an alternate route. This update is critical if you are using redundant gateways connecting to different mediation servers.