"Total Rx: 308 Lost=9 maxLossBurst=1 1 in a row: 9 "
This tells you it's the very occasional packet going missing and probably *just* enough for a caller to notice sub-optimal voice quality, but not enough for them to report "can't hear the other person" (for this call anyway). You probably want to observe the stats for a call that has been complained about.
From this, I would assume the network was (mostly) sound, but that there is congestion occurring somewhere.
As advised above, you should first figure out whether it's the same for internal and external calls - this might help you narrow down the controller/server/switch or router interface where the intermittent congestion is occurring.
And ultimately, this is quite possibly an issue you could solve by ensuring you have either QoS or traffic-shaping configured on the relevant device.