I saw that on last WWDC Apple announced launch of CallKit on WatchOS. After that part of CallKit framework becomes available for WatchOS 9+ but there is no any specific documentation or examples for making CallKit works on WatchOS.
My question is: is it possible to recieve VoIP pushes and accept VoIP calls on watch as for now?
yes it is; one such app that already does this is Wristcam (they make apps and also hardware, but you don't need their hardware to use their free calling app)
all of this is possible without a phone at all: the phone could be left at home or powered off entirely. your watch will ring or vibrate from a Wristcam call
it's even possible to do one-way video call (with two-way live audio) between your friend's iphone and your apple watch
