SNAT not working, return traffic stills goes to the orginal IP(H.323 packets)

Hello,

I'm trying to do figure out the following, we have a Avaya office PBX on a external location, between us and that location there is an IP-VPN connection. Between the router building the IP-VPN tunnel and the phone we have a Sophos UTM 9.The Avaya office PBX only accepts connections from network A, so I created an SNAT to change the IP of the phone from B to A. This seems to work the only probleem is that the Avaya office PBX want to sent traffic back to the orginal IP B, there is still someting hanging inside the H.323 packet with the orginal IP.

Anny idees?

Thanks.