I'm testing out Skype for Business myself here with a VVX 600. I have multi factor auth set up on my O365 account so standard username/password sign in does not work (even with app password etc). The only way I can sign in to use the web sign-in method. When I do this, though, I have to sign in again every single day.
I have turned off all of the auto log-out/timeout options I can find -- even through powershell.
I am on the latest version of the phone software 18.104.22.1685