Hi Blake,
we're using Nagios with the awesome check_mk plugin.
FreeSWITCH is simply queried using snmp.
This way I can monitor and graph active calls/sessions, sessions per second, uptime. I wrote some small check_mk SNMP plugins for FreeSWITCH, if someone's interested I guess I can upload them somewhere.
Additinally we wrote some site specific check_mk plugins that simply query some stuff using "fs_cli -x" and parse the output (e.g. availabiility of some special callcenter agents that have to be logged in).
See
http://mathias-kettner.com/check_mk.html for documentation on check_mk plugins. If you know some Python it's really easy to monitor whatever you want. To get up and running with the whole monitoring stack there's
http://omdistro.org/ from the same guys, it's a complete and integrated package of everything you need to get a Nagios stack with all popular plugins running with minimal Nagios knowledge.
Best regards, Markus