With the users having issues picking up the calls sometimes, is the client in the foreground when they are doing this rather than running in the background? At certain times, when the call comes in and the client is running in the background, the user needs to click the Windows 10 tray bar notification (call incoming etc...) before being able to accept or decline the call in the client.
Can you elaborate mote on the losing connection? Any specific error messages? Is there a message when they are switching the softphone on or off? Worth noting that MiCollab for PC does come with minimum requirements and if they aren't met they can cause issues.
Martin