There should be an option to trigger a failover when the server has no active calls or genie sessions. I sometimes find myself needing to force a failover during period of normal utilization for maintenance, troubleshooting, or some other reason. I typically find myself constantly refreshing the badge status monitor sorted by "call status" and watching for all badges to be inactive so I can time the failover correctly.
We constantly have to watch the Call Status and try to catch low call volumes to shut down a VCG or VSTG or Failover our VS. When you initiate a shut down on a Cisco gateway, the gateway will not take any more calls and will wait until the calls in session complete before shutting down. Vocera should do the same for their gateways.