My wife ordered an $80 refurbished Android phone to start using Page Plus's $80 for 2000 minutes prepaid plan. The idea is to use the Groove IP app with Google Voice while on wifi and PP's minutes while away from wifi. If she continues with her typical usage, it will be cheaper than our current $30/month piggyback-on-the-inlaws-family-plan plan. If I remember, I'll post an update here after we get it set up.
Okay, we did it! We switched from $30/month to an Android phone with GrooVe IP for VoIP calls from the phone over wifi, plus prepaid minutes through Page Plus when away from wifi, both through a single Google Voice number.
The GrooVe IP app is pretty good. The call is clear, and it can be integrated into the native dialer. The bad, though: Sometimes it hangs at the calling screen, and the phone needs a reboot. On outgoing calls, there is a two-second delay before the person on the other end can hear. This is irritating, but we don't know yet if it will be a deal-breaker, or if we can fix it.
For outgoing calls on the cellular network, the separate Google Voice app lets you call from your Google Voice number, that is, have it show on caller id. This works without wifi or data access, just cell. So both incoming and outgoing calls, on wifi or the cell network, all work under the Google Voice number.
For incoming calls through the Google Voice number, it's set up to forward to both GrooVe IP and the cell phone, which is a little weird to have the incoming call come to your phone two ways at once, but GrooVe IP is designed to put its answer screen back on top after the cell phone side starts ringing.
The battery usage might be an issue. We've only had a day to test it, so we can't tell yet. We're still working out the bugs, and I hope the GrooVe IP app gets better over time. I hope Google Voice continues to work for free for a few more years. As long as it's less than 4 cents/min, this setup might still be worth it.