No Audio problems between equipment

I’m going to try my best to be short simple and clear.
I’m an old school telephone man and current data network designer and installer, layer 2 only, IE I make it happen and fix it onsite.
I have a custom setup for a customer that is as follows.
A Raspberry Pi running RaspiBX 16 on an air gap network setup.
28 endpoints are connected via SIP and only work as legacy chan
I need one new endpoint ( a phone ) to answer when any of these endpoints are activated (they hotline to a que where the phone is the only member)
All of this is working fine except!
There is no audio when the legacy device calls the DN of the phone but the call rings and completes
The phone errors 503 when dialing the legacy device and return busy
The phone (multiple manufacturers tried) will only login as PJ chan device.
What am I missing here? There are no ports to be blocked as its peer to peer and air gappped. The RaspBX is on network and within the gap.