[cisco-voip] Low IP Phone MOS Values

[Andrew]

Dear All,
I am currently troubleshooting issues with very low MOS values being 
displayed on the IP Phone statistics. While the customer isn’t noticing 
any issues with voice quality, I’ve been asked nevertheless to attempt 
to resolve the issue.
There are around 20 IP Phones (7940/7961) in use on the site, and at any 
given time 2 -6 of them can be displaying low values, usually the same 
culprits every time. The phones are using the G711 codec and the 
switches are connected (with 1 exception) via gigabit links.
The interface counters on the phones and the switches between the 
phones, gateway and call manager (retracing the entire path) don’t have 
any more than a normal volume of errors (>1%). QOS is configured on all 
interfaces.
We made test calls both internally and externally with 5 separate phones 
attached to each of the switches, and every time the MOS results were 
between 4.48 and 4.5.
Values for the phones with issues look like:

MOS LQK 2.0000 Avg MOS LQK 2.2219 Min MOS LQK 2.0000 Max MOS LQK 4.1779
MOS LQK Version0.95
Cumulative Conceal Ratio 0.5963 Interval Conceal Ratio 0.0860 Max 
Conceal Ratio 1.0000
Conceal Secs 140 Severely Conceal Secs 133
Latency 0 Max. Jitter 19

However at some point the values are going back to the same low values. 
We enabled CMRs for the calls to determine if specific calls were 
showing the poor values, however for all of the calls being logged the 
media info is still carrying only the values “null”.

Does anyone have any advice on how we could try an resolve this issue, 
or if not the MOS issue than the null values for the CMR records?

Thanks for any help anyone can offer,

Andrew Cook