There is no concept of outbound port forwarding. The only thing the MX does is first create an outbound flow between your phone and the external server and perform SNAT on the packet. (so the source IP of your phone is translated to the WAN IP on the MX) Destination address is unaltered. It is the job of the phone to make sure it has the correct destination IP of the SBC it has to register to. The only thing you can manipulate is the outbound WAN circuit for that traffic. SIP ALG cannot be an issue since the MX simply does not implement it. I also take issue with the explanation of the phone provider. Proxy authentication required logically seems like the phone setup has not been done correctly and has nothing to do with possible packet loss. Proxy auth would be that the phone cannot directly authenticate using that server directly but has to go through another device. If that is not the desired behavior, then something is wrong in the config of the phone system.
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