Some WiFi routers have a configuration setting enabled by default that causes the calls to disconnect after 30 seconds. The setting causes the router to rewrite the Voice over Internet Protocol (VoIP) packet information incorrectly which causes the transport to die.
You can fix this by changing your router configuration to disable SIP-ALG
Check with your router manufacturer for details on how to find and disable the SIP-ALG setting on your router.
Fongo is designed in such a manner that we do not require any SIP-ALG logic to be interjected by systems to allow calls to connect or to be received. Routers with the correct SIP-ALG logic will work right, but many routers on the market today have an incorrect implementation that breaks VoIP.
You may be able to circumvent this behaviour by using a free anonymous VPN service Opera VPN
You can download the Opera VPN Service Below.