Screenshot

Currently the phone notifications (which should be reworded to Call notifications) options are set to

None

Incoming call

Outgoing call

Both

It would be great if you can add another option

Incoming call if silent mode

That is, it will show incoming call notification on your other devices only if the phone ta set to silent/vibrate. 

This will be good for the battery and costs as there will be fewer notifications. Also, the incoming call usecase is more helpful when your phone is on silent mode and you're working or you're in some other room.