This is a large topic, ans the answers will depend on how your network is set up. I would suggest starting by using the SoF Network Assessment tool at different places in your netowrk to determine where the Jitter is coming from.
If you can connect with a wire as closely to your internet connection as possible how is Jitter across the internet? If this is high then what can you do to improve the connection from the ISP, is it just not big enough.
Then use the wire again into the same point as your wifi access points are connected. If this adds much jitter what happens in your network between these two points. Are there network security devices that are well known to add jitter, can Skype traffic bypass any inspection rules.
If the jitter only happens on the wifi connection, what standards are your access points, are they 802.11ac, do your clients support this speed, and other roaming type extensions. Do you have enough APs to cover the number of clients, if the wifi channel getting interference from other souces, can oyu change that. Are you using 40Mhz bands as these tend to increase jitter.
Then on the client, is it only certain clients, what wifi adapters do they have, are their drivers up to date? Any other client security products that might be interfering?
SoF Network Assessment tool is a valuable tool, as is using CQD and Call Analytics to discover and rectify issues. It can be a lot of work however, we are WiFi only with 7000 Skype voice CloudPBX users, so this tuning became a huge mission, and led us to change lots of elements of our network, replace hundreds of access points, bypass firewalls, implement QoS etc. Now our Latency and Jitter are well within targets.
