The GRNET Real-time Services Team has developed a Nagios plugin that checks VoIP reachability at the application layer, by utilizing the CLI voip application simpleopal which comes with the OPAL (Open Phone Abstraction Library, the OpenH323 successor)

1. local gatekeeper endpoint registration
2. dialplan resolution of target number
3. reachability of target endpoint or gateway

At GRNET, we utilize this plugin to check reachability of all our VoIP connected institutions (about 15) and each one of their supported prefixes. The check calls are carried out 4 times a day and targeted towards fax machines located at the institutional PBXs, behind the institutional VoIP gateways. So this check truly reflects the VoIP reachability from the centrally situated Nagios server (NREN) towards all the edges (institutions). However, it gives no assurance of inter-institutional reachability, which is implicitly checked. Here is a sample screenshot of our Nagios page.

The plugin executes simpleopal with trace enabled and using the Expect perl module searches the trace output to determine the end reason of the call.
Here is a sample of a trace output.

check_voip_opal [OPTIONS]
  -n number     : number to check
  -g gatekeeper : gatekeeper to register
  -u username   : username to use for registration
  -t timeout    : timeout in seconds
  -p listenport : port to listen for H.323 requests

* options in bold are required