We are having call quality issues in the form of audio distortion during a small percentage of Lync calls, usually when a PSTN caller is involved, where the sound will start getting distorted. It can be heard in the form of excessive static, delayed sound, or (most commonly) a distorted voice that sounds robotic-like. Sometimes its minor, and other times you have to hang up and call back. We have Aastra 6725ip phones for most people, but have replicated the issue on a Polycom cx3000 conference phones. There are two different Lync standard servers in the mix, and the users are distributed randomly over the two servers, with no difference in possibility of experiencing the issue. It doesnt matter if the call started inside or outside the building.  We have a Net UX2000 gateway with 3 PRIs.  We also have a Lync monitoring server running, but on all the the calls that the users report with degradation the call report shows normal call quality. I have a ticket open with Microsoft support for over a month, but so far their service has been abysmal.   They have really done nothing but asked for logs, which I have plenty of.  I've checked network performance and from everything I can tell there are no latency or dropped packet issue.  We checked with the phone company and there are no issues on their end.  At this point our users are wishing we had stayed with Cisco.

I my experience these things can happen when using the wrong network cable. With wrong I mean, those phones are somewhat picky about the cables. I have seen it a lot the the Aastra 6725IP giving quality problems and was solved with a new network cable. We don't use any CAT5e cables anymore between the phone and the outlet, only CAT6. Maby this could solve your problem as it has for us.

I'll give it a try, at this point I'm open to anything.

Any update?

Check whether the background noise affect the audio quality as in the following thread.

http://social.technet.microsoft.com/Forums/en-US/ocsclients/thread/4c043ba8-1229-478d-85e3-7dccee7bb99f

Still having the issue even though we changed out the cables.  Any other suggestions?

Does this happen with different people or always the same people? Even if the monitoring is saying the quality of the call is normal is does not mean it is. Check a call that is reported and a call that had normal sound. Compare the reports with each other maybe something with codecs or anything else. Also check the UX gateway, I am very familiair with that gateway and I know it has extensive log tools, maybe there is something there (as you say when this happens it useually involves a PSTN call)

I have an open case with NET, but they haven't found anything.  Typically when these distorted calls happen the PSTN caller doesn't hear it, it's only on the Lync side.  Yes it happens randomly to our users, but we do have a few users who are more vocal about it.  We were able to recreate it on a call late last week and do some testing.  We found that it only happens with the phone client and not the software client.  Also we noticed that if we turned off the EnableClientMusicOnHold option in the client policy it did not happen.  We have turned this off on a portion of our users to see if it helps.  I also submitted the call reports from this testing to microsoft, but it's been silent on that front. 

I do know that I also had problems with the MOH and the NET UX2000 the MOH function is also an option within the NET. Maybe if both are enabled this problems occurs, but not sure.

We tested that scenario as well and it made no difference, the only thing that helped was turning off the MOH on Lync.  So to provide some sort of MOH we turned it back on in the NET gateway.

We still have call quality problems even when we don't put the call on hold. 

Did you resolve this issue?, because we have the same problem in our production environment and all the calls with robotic sound it presented only in the inbound flow of calls to Lync, all calls in the outbound sounds perfect.