[asterisk-users] Odd one-way audio problem

Antony Stone Antony.Stone at asterisk.open.source.it
Tue Mar 19 18:19:08 CDT 2019

On Tuesday 19 March 2019 at 21:36:53, Mike Diehl wrote:

> Hi all,
> I have a user who is reporting one-way audio, but only when a call is made
> to or from particular PSTN (cell) numbers.

I'm assuming you're using a PSTN trunking provider to connect to those numbers 
(ie: you don't have your own on-site gateway device).

Do you use only a single trunking provider, through which some calls show this 
problem, but most don't, or do you use several trunking providers, and the 
call numbers showing this problem always go via the same one?

> Their phones are behind a NAT router and my server is on the open Internet.
> Calls within their office sound fine.  Calls to/from most numbers sound
> fine.

I think that basically rules out the common NAT reasons for one-way audio.

> When they took their phones home,

...almost certainly also behind NAT...

> those same phone numbers still had problems.

But presumably other numbers didn't?  (Important to check!)

> So, I don't think it's their network.

From what you've said, I think you're right.

> I've taken pcaps of both legs of example calls.  On the provider-side, I see
> 2-way audio.  On the client-side, I only hear one side.

Please explain that in a bit more detail.

You have an Asterisk server on the Internet, presumably with one IP address 
(or maybe two, but one IP4 and one IP6).

Where are you capturing "the provider side" and "the client side"?

Can you show us the tshark / tcpdump / whatever commands you are actually 
using to perform these captures, and make clear which machine/s you're running 
those commands on?

> Most of the time, though, their phones work correctly.
> Any ideas where to look to fix this?

Only two things spring to mind so far:

1. Transcoding?

2. IPv4 on one side and IPv6 on the other (although I'm hard pushed to see how 
this could create one-way audio rather than no audio)?

I think the key thing I would look for in the pcaps is for any re-invites - is 
one side telling the other "oh, you can get my audio from here" and that's not 
an accessible address?

However, why this would be specific to particular _numbers_ rather than 
particular SIP connections puzzles me too.


Please apologise my errors, since I have a very small device.

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