I have Avaya 96xx series telephone handsets that I want to connect to a remote Avaya PBX across the internet using a Draytek 2760.
The connection uses the h.225 ports 1719 TCP & 1720 UDP as well as UDP media ports 40,002 thru 40,256
The phones never connect.
Firmware is 3.8.2.3_BT

If I use an alternate router everything connects fine, I have used TPLink Dlink and Cisco routers, all work perfectly.

Working with one phone: If I create a port forwarding rule for 1719, DMZ the phone and create an inbound "allow" rule for the remote phone system it works. If I remove any of the 3 options it stops.

I am connected to the internet using WAN2 and an FTTC modem. (Note: I get the same issue using the internal VDSL modem) The WAN2 connection means I can port mirror the phone (LAN1) and the WAN2 connection to LAN3 and run wireshark .

The problem appears to be that traffic on port 1719 goes to the router then to the phone system, is returned to the router and then is not passed back to the telephone handset.

All the other ports traverse the NAT just fine once 1719 is forced.

I have tried with sip_alg both on and off.

I have reported this to Draytek but have not had a response in 2 weeks other than a confirmation.

Has anyone come across this?
Does anyone know how to fix it?