What's your phone Interworking profile in your endpoint/subscriber flow? Is that interworking profile picking up on Avaya-specific extensions? Does the application relay for PPM/HTTPS being proxied thru to SM and back to the remote worker operate?
NATs and SIP aren't exactly new. I could only believe the phone is stupid enough to offer a private IP because of a misconfiguration or lack of configuration of telling it where to route audio. Last time I had that problem, the phone was sending RTP to the private DMZ IP of the SBC and not the public IP because of that public IP override setting in the SBC.
Either some interworking profile isn't passing the phone the intelligence it's looking for, or it might not be getting PPM. Is it AST registered in SM? Is it the same deal from a Windows Equinox client? I know in the Windows ones there's a "send logs" that just attaches them in an email. iOS probably has something similar and I'd sniff through those. It's not terrible if you find the right log file. Something like "I really don't think I'm behind a NAT"
Wait...I'm thinking this through on the fly... Now, it is a fact the SBC needs the audio from the remote worker first to know how to get through your NAT. When I've looked at good calls, the audio sets up something like 1 RTP packet from client to SBC which the SBC hijacks to send audio back to the client and I guess thru an update message with SDP tells the client to send audio to the same IP

ort+1. I'm not entirely sure of the mechanics there, but that's the idea.
Now, I don't actually know for a fact if that private c= line is the culprit. For all I know, the SBC knows how to tell the phone to reach it, and it uses its brains to get back to it regardless of SDP content.
Can you traceSBC RTP to 192.x out B1 on the dead air call?