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Loud static when pbx does not receive RTP


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We're having an issue during calls.


The issue happens when we make an outbound call from any of our registered endpoints (Polycom/Panasonic/Snom) out our PSTN trunk, and we reach a voicemail system on the PSTN.


After the "BEEP" tone that voicemail systems use to indicate when to begin recording a voicemail, there is obviously no sound being transmitted from the far-end voicemail system to Snom ONE.



During this time that we are recording a voicemail, there is an extremely loud static/gargling sound heard on our endpoint.


This ends as soon as audio begins to be transmitted again from the far-end.



So, we heard this static sound ONLY when Snom ONE is not receiving RTP from the far-end.




How can we disable this behavior??



I had opened a pbxnsip TT on this quite a while back and never got a definitive reply.

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There are two possibilities here: First, if the PBX does not have to send anything, it sends comfort noise. This comfort noise can be relatively loud as we recently found out, bug not the "thundering" kind of white noise you get with SRTP (supposed to be "comfort"). Workaround: Make sure that your external voice system sends RTP all the time or ask for a version from snom ONE with a fix (we have one available).



The second possibility is that there is a SRTP problem; old firmware versions had that problem, but if you have something younger than two years the problem should not exist any more on snom phones.

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