How to make Siri-initiated calls use Google Voice
Google needs to integrate Siri into their Google Voice app. Only if Google makes the integration you can use Siri with Google Voice. "Call Alice with Google Voice" is a good example.

Audio or video calling integration with Siri is possible since iOS 10 (September 2016). And for developers to build the integrations since June 2016.

So my best advice is, start asking Google to build this integration.