How to check if extension (phone number) is reachable?

I would like to check if some phone number is reachable from Cisco UCM. Another member of my company is getting the error "Redirect destination is out of order" when trying to redirect the CiscoConnection to another phone number (to the voicemail). I would like to check if the voicemail number is currently reachable. How can I do that in the JTapi code?

I'm not aware of any programmatic way of validating that a call can be completed other than actually attempting the call and getting success/failure. The CUCM admin can use the Dialed Number Analyzer to investigate the situation manually, however, at least with regard to the dial plan configuration - ephemeral reasons for call failure (devices/trunks going out of service, for example) could still happen.