It's no speech at all, so lets call that 2 way.
I don't think it is a NAT problem although i wouldn't rule out some other networking issue.
From what i can see, from the unsuccessful "Telco 2" trace, in the packet immediately after the Invite,(100, Trying) the PBX sets it's Contact URI to number@10.13.154.157.
I would have expected number@192.168.13.13
The next packet (Status 200 OK from the PBX) repeats the number@10.13.154.157 Contact URI
The packet after, (Request ACK.... from Telco 2) is addressed correctly, number@192.168.13.13
Bu from here on, its all one way RTP traffic from the PBX.
There is no reply from Telco 2 as although the packet Destination address is correct (Telco 2's SIP Peering server), the Source address is 10.13.154.157
10.13.154.157 belongs on Vlan 25, not the native VLan, so why is FPBX issuing this as the Source address for SIP traffic on the Native VLan ?
10.13.154.157 only appears in the config in two places that i can see
It's the defined Static IP for Eth 0.25 and the Override External IP as previously mentioned.
I'll take a closer look on Monday and see what i can find.
In the meantime i'll take any suggestions.
Jon