Troubleshooting Client Connectivity Issues
The first step is to enable full logging in the Skype Client itself and then check the log on the client side. In Windows this normally means going to the Skype client "settings" and then "Enable Full Logging".
After that you can open Event Viewer, click on the Application Log and see what entries for "Lync" come up (yes it is known as Lync despite the fact the product itself is now called Skype 2015 or 2019 for business).
Further Troubleshooting on the Front End Server Pool or Edge Server Pool
The first step here is to really just check th "Lync Server" log under "Event Viewer" and the "Applications and Services Logs"