Some Wi-Fi routers have a configuration setting enabled by default that causes the calls to disconnect after 5 or 30 seconds.
Solution #1
You can fix this by disabling SIP-ALG in your WiFi router's configuration.
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.
Solution #2
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.
Solution #3
Enable "Alternate Fongo Connection" in the app's settings.
Solution #4
You may be able to avoid this entirely by using a VPN service such as TunnelBear (free) or NordVPN.