The following undocumented command is useful for debuging H323 messages. This command cannot be found in the help-context of IOS as noted below.
router# debug gatekeeper main 10
router# debug gatekeeper ?
gup Gatekeeper GUP messages
load Gatekeeper load-balancing messages
servers Gatekeeper Servers
The great thing about this command is the simplicity in the debug output.
Click the screenshot for a sample output.
