I am getting this error while running 'Remote Firewall/Nat test utility'. Can anyone has idea ? VOIP traffic from this computer or network cannot work in Stun Mode. Possible Causes: 1. Check your Firewall Configuration to confirm that your firewall is not blocking packets 2. Check you Internet Connection to ensure that you can reach the stun server. Try and ping the stun server host. 3. Confirm that you have performed the 3CX Phone System Firewall checker and that the test passes completely. To do this access the 3CX Management Console and go to General Settings > Firewall Checker > Run Firewall Checker.