This problem may be related to Microsoft Firewall settings. To resolve this you should go to the Windows Firewall settings window
Start / Control Panel / Security / Windows Firewall and click on "Change settings" label. Choose the "Exceptions" tab.
You should make sure that "pc2phone" and "VoipTunnel" entry are on the list and their checkboxes are ticked (as shown above).
If the checkboxes are unchecked you should check them with a single mouse click and then confirm by clicking OK.