From Android Central:
"First, you're going to need the new Version 2.3 of Google Hangouts, which is rolling out this week in its slow, safe, rollout fashion. You're also going to need to opt in to "migrate" Google Voice over to Hangouts... And to make phone calls through your Google Voice number, you're going to need the new Hangouts Dialer app, which also requires that new v2.3 of the Hangouts app. Things get a little funny here, because once the dialer is installed and you open Hangouts or open Hangouts Dialer, they look and function exactly the same. So pick either one you want on a home screen. Doesn't matter which."
What a mess, this doesn't make me regret ditching Google Voice one bit. At least Google's actually making changes rather than neglecting Voice entirely.