Some Wi-Fi routers have a configuration setting enabled by default that causes the calls to disconnect after 30 seconds.
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 SIP-ALG enabled. Routers with the correct SIP-ALG settings will work right, but many routers on the market today have an incorrect implementation that prevents VoIP from working properly.
Alternatively, if you cannot find the SIP ALG setting, you can log in to your Wi-Fi router's web page and enable port forwarding on ports 5060 UDP and 13000 UDP.