Model :

Hardware Version :

Firmware Version :

ISP :

We experienced one way audio using TL-MR6400 router between a SIP PBX on our LAN and the SIP trunk available on Internet.
We discovered with a Wireshark trace that the PBX sends as SDP in the 200OK:
"Media Description, name and address (m): audio 16566 RTP/AVP 8 101 13
Connection Information (c): IN IP4 151.1.170.11"

So the PBX discovered its public IP using Stun. The provider is also expecting packets from this IP-address 151.1.170.11, so this should work.

In the trace from the other site of the LTE connection (italian operator Wind), we also see the 200Ok from the PBX. However the SDP contains now a changed address, namely the private-IP address 172.16.0.172:

"Media Description, name and address (m): audio 16566 RTP/AVP 8 101 13
Connection Information (c): IN IP4 172.16.0.172"

Since 172.16.0.172 is the external IP-address of the NAT-router, I suppose that it changed it.

Please provide a fix for this erroneus behaviour that happens with or without SIP ALG enabled on your router.