Right this moment traveling, no reliable internet connection, but here is my /etc/default/rtpproxy https://pastebin.com/ybc20Q8DAuth required is a normal part of the handshake; it is the proxy telling your phone that a password is required to make calls. Your phone replies with the username/password and the call proceeds. In the sngrep, take a look at the INVITE that is going from the proxy to the called phone. What IP address is being offered in the SDP portion of the INVITE (the lower portion; the IP address is on a line that starts with c= ) ? That is the IP address that the called phone will send the audio to. If it's the IP of the calling phone (that is, you are not using rtpproxy/rtpengine on your proxy to relay traffic), you need to make sure the phones can communicate directly with each other without being blocked by a firewall or NAT. If it's the IP of the proxy, you need to make sure rtpproxy is running so that your audio traffic is relayed.