Outbound external calls, no sound at receiving end

Hello, since a month or so some calls to external cell phone numbers have that problem that the other party cannot hear us, but we can hear them. That seems to happen only to some numbers, for example I cannot reproduce that calling my verizon/xfinity phone, but when calling my colleague with phone in Tmobile, it happens randomly, every second or third call.

I have captured and reviewed pcap on calls that have that problem, and calls that dont and wasnt rly sure what I am looking for. Interesting finding was on the router, checking data flow connections. It shows that when the call is good on both sides, there is just one RTP connection with external media server made, it is always server of ip similar to 206.147.84.120, connecting to our freepbx server on some port between 10000-20000. However, when the call has the problem, and receiving end cant hear us, there are 2 RTP flows with a media server, from which one tries connect to our freepbx on port like between 20000-60000. Our freepbx is configured with default ports 10000-20000. That looks like it tries to open separate RTP flows for incoming and outgoing data? I havent tried to edit RTP Port Ranges in the freepbx settings yet.

this is a single flow when call is all right and both parties can hear each other. Port 13456

this is when receiving party cant hear anything. There are 2 flows, from which one with port 52727

Again, this issue happens only when it is outgoing call, only to external number, and as mentioned, not all numbers

I am guessing that the PBX sent out RTP before receiving any, and the router/firewall rewrote the source port to 52727. The media server may have discarded those packets because it expected them to come from port 11978.

Confirm that you have UDP packets with destination ports 10000-20000 forwarded to the PBX, allowing any source IP address and port.

Confirm that the router is not intentionally rewriting the source port. There is no standard name for this; look for a setting such as ‘disable source port rewriting’, ‘disable source port remap’, ‘disable source port randomization’ or similar.

It’s also possible that the router had to rewrite the source port, because UDP port 11978 was being used by another device at the time. Do you have any VoIP apps that do not go through this PBX?

sure NAT is in place. It started to happen not long ago. And why just with some numbers. Never happens when you call verizon - the media server is same, and one rtp flow is made

this is sangoma SIP so I opened case with them to get some hint maybe

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