I am guessing that you mean "one of your local VOIP extensions cannot call another VOIP extension". Can you be more specific about what exactly happens?
Couple of things to check: Both extensions are green on the line status page, otherwise you have a registration problem with the one which is not green. Also, make sure that the extension that you are calling and is not responding is not set to forward the call to some undefined destinations.
If you can supply more detailed info about the failure, you would get better help. Your current message calls for shooting in the dark.
Halea