There are many variables that can either cause latency or impede server to server traffic supporting Unified Voice (UV)
To avoid latency caused by packet inspection of SIP traffic UV, simply be sure to disable SIP packet inspection on an application-aware firewall. The best-practice it to implement a global address tag in the sip.xml file on the Flash Media Gateway (FMG) server in [root_install_dir]/conf/sip.xml. This will also help with traffic blocked when FMG is deployed behind NAT. Here is the tag in sip.xml under profileID sipGateway
The global address variable can be either the IP address or fully qualified domain name of the Flash Media Gateway externally.
FMG and SIP works best with Connect when there is an absence of superfluous speed-bumps; click on this diagram thumbnail to view the traffic flow among the servers: