I have disabled the tick box under Settings > System > Notifications > Show reminders and incoming VoIP calls on the lock screen.

However, when the screen is locked and someone makes a skype call it still rings. Furthermore, if I set the volume to mute, for some reason that doesn't work and it reactivates.

I have tried using the reg fix solution located here and it didn't affect it either.

HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Notifications\Settings

This is problematic because we sit in a shared workspace and if I'm away from my computer and activate the lock screen and someone calls me on skype it still rings loudly and disrupts co-workers.