Incoming call audio issue

[One]

Hi,
I have a problem when phones receive a call, for a time varying between 1-3 seconds, they hear nothing (at the beginning of the call). The problem does not always occur, but with a frequency of 2/3 time every 10 calls. After a few seconds the conversation presents no further problems.
Listening to the captured audio the call doesn't seems to have any problem, the audio is complete, but when they answer the call the problem is there and they can't hear the first 1-3 seconds of the conversation.

I have a FreeSWITCH 1.6.20 on Debian 8, installed from repo.

Has anyone already encountered this problem or would you know how to help me solve it?

Best regards

 

[Adrian Fretwell]

It's difficult to guess at what may be your problem, but to me it sounds network related. Do some analytics on your network and look for lost packets, out of sequence packets and excessive jitter times. Wireshark provides some useful audio (RTP) analysis.

You say that recordings of problem calls sound fine, this may be because, 1. the recording mechanism is more tolerant of jitter / sequencing than the end point, or, 2. the problem is occurring between the PBX and the endpoint, so the PBX doesn't see it. Depending on your endpoint you can make some manual adjustments to the jitter buffer like min/max delay etc.

Because of the asynchronous nature of most internet connections here in the UK (high bandwidth down, low bandwidth up), I have encountered many more problems with upstream RTP than downstream RTP. If possible we always implement rules on the customers router to ensure some bandwidth is reserved for the RTP, especially on the upstream.

If the endpoint is behind NAT, the endpoint will not receive any audio (RTP) until it has sent some which will open the NAT hole. I have seen endpoints delay the start of RTP transmission while some other negotiation/process is going on.

 

[One]

All phones and PBX are in the same local network, the PBX is virtual instance on MS Hyper-V.

I will try to do further analysis to understand the cause.

Thank you for your feedback.

Best regards