Our firewall contains all of the IP's/domain names of our customer's. No one else is allowed to connect to the pbx.
Since the firewall is now correctly only letting our customer connect, the issue has become that audio is missing from one side.
Turning off the firewall makes the problem go away, so it's definitely a firewall issue.
My question is, WHAT is being blocked and why?
Customer phones are able to connect remotely, they can generally perform dialing functions, they just can't hear one side of the conversation.