It took me a bit to figure it out, but the settings are right there in Google Voice. Just go to the Settings, Phone, then click “Activate Google voicemail on this phone”
It will have you type in the following number, where xxxxxxxxxx is your phone number.
So, if your phone number was (555) 123-4567, you’d type in:
Then hit SEND.
Ensure you’re dialing out with your real phone number, not your Google Voice, otherwise it won’t work.