Following its latest iOS update, Google Voice now works with Siri. You can use Apple's assistant to start calls or send messages through the service. You can enable Siri control by going to the Google Voice options and marking a default account for calls outside the app. Then, you'll have to activate the Use with Siri option for the app in the iOS device settings.
It'll surely be a welcome update for Google Voice users who have an iPhone or iPad, particularly since Siri can learn your calling and messaging preferences, but it leaves the service in a strange place on the voice assistant front. Sure, it works with Siri, but you still can't send messages or start Google Voice calls with Google Assistant. Perhaps the Siri support is a bellwether for Google enabling Assistant control for the service in the near future, but for now, Voice has a curious dichotomy going on.