If you have no incoming sound while in call, it probably due to the fact you are under a NATed network and your SIP provider doesn't manage that.
In order to workaround this case, it's sometimes useful to enable the STUN option.
If you have no incoming audio problem ignore this warning.
If you are in a country where public network is separated from other public networks, you may have to change the STUN server.
Use of STUN consume more battery.