"User not found" usually indicates that either
1) The obvious: That extension doesn't exist
OR
2) The not so obvious: The dial plan has not been configured properly and is causing calls that should go through on an outside/VoIP line to be routed internally instead. This applies especially to users who are using ET in combination with Axon.
Note: The 4 tones are generated by Express Talk itself to indicate a general error. When you hear those tones do not assume that the call was taking place and hung-up after the tones, the tones are the result of a call being hung-up or not taking place at all. Even if you are completely disconnected from a network, Express Talk will still play those tones for every attempted call just to indicate something went wrong.