2 FreePBX systems, calls OK between, one way audio once transferred

So, we have a situation where we have a number of FreePBX with SNOM handsets installs out there and we have a FreePBX with SNOM phones. If someone from one of the other FreePBX installs calls us, we can answer the call and we have 2 way audio. However, if the call is then transferred between extensions, we get one way audio (we can hear them, the originator of the call but they can’t hear us). Also, just to add fun,. if we try and park the call, the attendant announces the slot the call is in then promptly drops the call. Other than that, calls from any other phone/ phone system can be transferred, parked etc no issues… for us it is just FreePBX/ SNOM to FreePBX/ SNOM thing.

We have set all extensions on 2 of the systems to NAT=YES. Most of the systems use SIP trunks from Comwave and in the SIP setup we have NAT=YES…

Any ideas are appreciated.

You shouldn’t need to have the phones set as NAT.
You only need that if a phone belonging to the pbx in question is behind NAT.
Your phones are local to each pbx which is supplied a trunk from Comwave so all the handset are in fact local to each pbx.
I would be looking at RTP on each box. Make sure ports are forwarded on your firewall to your pbx at each location. default ports are 10000 to 20000 but you can change that if you don;t want to open that many ports. Remember you need 2 or 4 ports PER SIMULTANEOUS CALL forwarded.
See how that goes.