Problem of Cisco 7920 VoIP phone on Cisco 870 router

I have a Cisco 7920, a WiFi VoIP phone, connected to a terminal of Cisco 870 router. Everything is on a remote site connected via SDSL lines. This remote site configuration is repeated over 50 times (include more than 50 remote sites).

My problem is that some phone has a background noise, sound very low and everything behind a terrible crackle which makes communication virtually impossible.

I tried to replace the telephones in 7920, but the problem returned and most phones do not seem to have the problem when connected elsewhere. I tested the router, but nothing seems malfunctioning because they are also up for a data network that has no problem.

So, if you have any ideas for my problem then please help?

It's not really my field, but it looks much a problem with audio codecs. Not knowing all the material used, I can not help you more. Finally it is possible to change the codecs used, try the GSM codecs little greedy in terms of speed and reasonable quality.

To solve this problem:

1. understand how it works (how it is supposed to work)
2. identify each element coming into play,
3. carry out tests designed to eliminate the influence of a one after another,
4. once the element found at fault, make the necessary correction.

We see the importance of
- Codec (at each phone)
- Wifi connection
- Liaison SDSL
- LAN feature: switch and every element 100M advised 802.1Q if possible.

Have you purchased these materials or they were provided to you by a provider? I suppose you have a provider for you who proposed this solution. So it is his responsibility to solve this problem.

So I tried testing with two telephones on a site and the problem was the same. So I reached to the conclusion that the problem came from wireless router. After resetting the router to factory default and reconfiguring it, everything seems to be back to normal.

Anyways, thank you for all these explanations.

Sorry didn't get a chance to look at this post earlier. Had similar experience before. It was resolved, by resetting everything. I was using the 871 in combination with UC520.

Thx,