I am building a free voice call application on Android. I used OpenSIPs open source to make a SIP server and successfully make a call to another phone in WIFI network. However, I couldn't make a call when the phone uses cellular network like 3G/4G. I found out that I should make a STUN and TURN server to find exact SIP address.
Someone told me that it is hard to find SIP address if the user uses 3G or 4G network. So, he said I should use SBC to redirect voice data if I want to make good connections. He said I should make redirect servers to have good quality of service.
Is it possible to make a good voice call service with OpenSIPs server? If it is true, is there voice call services who used OpenSIPs open source? I've searched all documents, but couldn't find the answer.
Thank you.
