NN3820 not passing traffic to SIP provider properly, only receiving -Help needed determining cause

I have 2 NN3820s set up in HA pair, where I'm trying to turn-up SIP trunks with AT&T Flex Reach.  Currently I have only the primary connected to the ACCESS side (SIP provider), and both connected to our CORE network.

When calling to a test phone number given to me by the carrier, setup on a test phone on our CORE side, I can see and capture the incoming INVITE message, and I can confirm via Wireshark that it is getting to the CORE side and to the telephone device, and the device is sending back a 100, 180, and 200 OK message, plus it even looks like RTP starts to send (I can monitor the RTP on the Wireshark trace when tapping the internal side of the call)... but I am unable to get any SIP traffic showing back