For additional information, we have the same issues with Skype from phone to phone, in a dedicated VLAN and WLAN with the suggested settings above, ZD version 9.6. The communication with skype degrade slowly until we have no audio signal anymore.
We also noticed when the audio signal is strongly degraded, a reconnection of the phone to the wifi improves the quality of the communication until the interruptions restart again few minutes later...
If somebody has any idea, we begin to go blank !