Enabling Do Not Disturb in your Voice settings will automatically send incoming calls to voicemail.
1. After opening Voice, navigate to the "hamburger" stack button on the top left corner of the app. 
2. Scroll down until you see "Do Not Disturb", Tap the slider so it goes right and turns blue.
Your callers who try to call you through Google Voice will now be sent straight to voicemail.
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