Occasional one-way audio / no voice from external caller

[SKDamon]

Almost every day there are calls which when answered, we can't hear the caller's audio. This happens randomly on different endpoints. Sometimes the caller will try again and succeed. How can this be de-bugged?

System Info:
3CX 15.5.15502.6 on Debian 9 @ Azure (PBX Express setup)
3CX SBC on Debian 9 (3CX ISO download)
Encrypted Tunnel between SBC/3CX instance on default port
Firewall Check is all-green
VoIP Provider: EasyBell Germany Trunk

Incoming calls are signalled to a ring group w/ 3 members. Today, the problem appeared on a 3CX Windows Client as well as a GrandStream GXP2170 just minutes apart, other calls were not affected.

I have a network trace recorded directly with tcpdump on the Azure VM. In Wireshark, I can see+hear audio coming from our extension, while the external party sends RTP packets but the audio is completely silent apart from a large spike at the beginning (both failed calls I recorded show this).

The SIP flow shows that some party sent "SIP Status 415: Unsupported Media Type". The "To:" Header is set to the external caller, the "From:" Header is set to the internal extension. But the packet was captured on the lo interface I think, it's going from 127.0.0.1 to 127.0.0.1.

The caller sent a list with supported codecs in the SIP INVITE. I cannot see any Reply with codecs from the 3CX client, but there is a packet from/to 127.0.0.1 SIP Status 200 OK which mentions G711 PCMU/8000 which was then used in the 2 initial RTP packets. The 3CX Client sends some SIP INFO/MESSAGE with "chat support request" before the caller sends "415 Unsupported Media Type".

There is nothing in the SIP flow for the next 12 seconds, until the 3CX client sends "Sip0BYE".

I'm not firm with SIP/SDP/RTP, but I think the 3CX was missing out on sending a BYE/CANCEL to the 3CX client as soon as the external caller sent "415 Unsupported Media Type", resulting in a call with no audio from the external caller.

Would some kind SIP wizard like to see a SIP Flowchart from the call?

 

[cobaltit]

So you mention the firewall check is all green, but have you run it since SP6? There were port range changes in SP6 per the stickied thread.

 

[SKDamon]

Yes, I re-ran the check after the last upgrade. The "failed" calls used RTP ports below 10000.

 

[YiannisH_3CX]

Hello @SKDamon

I would recommend creating a ticket about this with our support department so they can look at the configuration of the PBX and a capture of the issue to determine what might be causing this.

 

[oliverhamburg]

Hi, have you been able to sort this out?

 

[YonNomNom]

Hey It looks like i am also experiencing this issue. We have the GrandStream GXP2170 too.

 

[YonNomNom]

Is there any update to this issue?

 

[SKDamon]

So far I haven't observed this behavior again.
My current theory is that those were calls where the external partner hung up before the call was established, EasyBell (SIP provider) had some issues or GrandStream still hasn't fixed the "SIP Status 415: Unsupported Media Type"-issue entirely in its latest firmware 1.0.9.108.

@YonNomNom Which firmware do you use? We had similar issues every 3rd call or so on the 2170 with the 1.0.9.102 officially supported (atm) release. See http://firmware.grandstream.com/Release_Note_GXP21xxColor_1.0.9.108.pdf

Which is why I haven't contacted Support yet, I'm using an unsupported up-to-date firmware with that bugfix included. What's weird is that this issue definitely occured on a 3CX Windows client too. I consider this issue closed (until it happens again, I'll let you know )

 

[YiannisH_3CX]

Glad to see the issue has been resolved and thank you for updating the thread. Hopefully you will not face the issue again

 

[YonNomNom]

@YiannisH_3CX do you have any eta on the release of the newer firmware from grandstream? It's been out a few months.

 

[YiannisH_3CX]

@YiannisH_3CX do you have any eta on the release of the newer firmware from grandstream? It's been out a few months.

As mentioned in a different thread, we(3CX and Grandstream) are testing a newer firmware version that the one currently available. Once tests are done and both sides are happy with the results the new firmware will be released. Unfortunately i do not have an ETA yet.

 

[YonNomNom]

@SKDamon Just wondering how you managed to update your firmware without 3CX putting the old firmware back on automatically? At this point I am desperate to fix this and I cant wait for 3CX to publish there approved firmware.