Basically, the watch will ring and vibrate on calls, even it is not on a user's wrist.
There is an option in settings called "only show notification while on wrist". In reality, this only works for app notifications. This setting does not apply to incoming calls. So whenever a call comes in, my watch on the table will go in full blast.
The only workaround I can tell is probably do not disturb mode, but I don't think this should be taken as a permanent solution.