We have had some quality problems with softphone VoIP (PC clients) after we upgraded from ZD1000 to ZD1200 (and also from firmware version 9.3.4 to 9.12.1). The problem was that at every 20 seconds there was some interference on the call. About two or three seconds of the speech was lost. The interference occurred only on one direction, on the speech sent from VoIP (on WLAN) to the other end (for example a mobile phone). The speech from mobile phone to VoIP was crystal clear all the time.
We increased the background scanning time on Ruckus from 20 seconds to 60 seconds. After that the interference occurred every 60 seconds, so it is definitely related to this. Later we increased it to 10 minutes to get rid of the problem.
So if your users are complaining that some words are missing from VoIP calls, try to increase the background scanning time, at least it worked for us.
The quality problem didn't occur at all when calling internally from one VoIP to another (pure UDP signal directly between two clients). The problem is somehow related to RTP traffic (which is the protocol of calls that go through the telephone gateway).