One-way audio through gateway (no NAT involved)

I've recently set up a fresh install of Trixbox to test out it's functionality, but I'm having one-way audio issues. The set-up is as follows...

POTS phones <---> Siemens Hi-COM <---> Sasmsung Gateway <---> Trixbox <---> IP Phones

This is a big site with thousands of users, so I've had to plumb in the gateway to connect me to the outside world via our old Siemens PBX systems. I've trunked 16 channels into the gateway, and then linked through my LAN to the Trixbox server and the IP phones. I previously had a Sip-X server up and running to test, and the audio on it was fine.

With Trixbox I am getting no audio transmission to the old PBX phones, although the IP phones work perfectly with each other and can hear the PBX users no problem. First though is always NAT or firewalls, but there are none. Second is codecs, but specifying and excluding codecs has had no effect. A debug on the gateway shows the correct SIP messages hitting the gateway in the right order, so I'm a bit stumped now. Anyone got any ideas where to look next?