I know I have not provided every detail needed to figure this out but I have provided a lot of information.
Yes, the issue is likely present on certain “networks”. I don’t have the ability to test with multiple addresses with an ISP’s network so I stated an exact problem. I do have the ability to test using my home network and a cellular connection and can reproduce the issue. Using the exact same phones and networks, connecting to two different severs in the same data center that use the same routes and firewalls, gives two different outcomes. One server results in no audio while another server has no issues, Same routes, same phones, same networks. I use the same SIP settings. Trunk calls have no problem so I did not look at direct media but it is not allowed. NAT is enabled and always has been.
I fixed this, but I would like to understand what is causing the issue to happen.