I don't think sprint mvnos have wifi calling.
Only option is to look into moving your number to google voice. You can setup cellular calls to look like they are to and from your google number and you can use hangouts over wifi using the same number
Ghoul, this probably doesn't work since you need to receive calls, but if google voice isn't an option, an app you can try for outgoing calls only via wifi is Vonage mobile app (not the Vonage Exetnsion app). You DO NOT need Vonage service. When you call out on the app, your cell phone number will be in the receivers caller ID. To receive calls and send/receive text the other user would need the app. The call quality IMO is equal/better than google voice and not materially different than a cell call.
Another long shot is to request a Sprint "Magic Box" to extend the sprint coverage - search the forum for a thread on it under "Future of Sprint?" for more details.
Another option is to request domestic roaming be added to the account. I don't get Sprint coverage at home but do get AT&T and Verizon strong signals so I use Verizon for voice and an AT&T Mobley unlimited plan to feed data to my CellNUVO phone. A mile or two from home the Sprint signal picks up again and I can use on network Sprint without issues.
Instead of SMS/Voice/Data rates of 3/20/20 for Sprint, I think it's 3/80/210 for Verizon, US Cellular and other carriers. You could always shut off cellular roaming data.
Another VOIP option is Skype. I have that installed on my Sharp Aquos Crystal.
Do wish they had Sprint VOIP but I've read the same thing that Sprint doesn't allow their standard VOIP app to be used by MVNO's but I know some use a different VOIP or SIP service (Republic and the late RingPlus would be two examples). I'd be tempted to use that internationally if CellNUVO added VOIP or SIP service using my existing cell number. Instead use Piranha Mobile prepaid international sim service or other options internationally.
I am aware of the 50% rule on most carriers. Not an issue since primarily use landline at home and data is mostly fed from the AT&T Mobley (neither would involve Sprint). Get the rare incoming call or 2 step website authentication SMS on Verizon. Roaming is mostly needed for Sprint dead areas in town or when I'm traveling.
Away from home tend to feed data from other sources like the AT&T Mobley, a Sprint Karma with way too much data on it or public wifi. This conserves my silver since I need 20-30 minutes of CellNUVO Sprint airtime on days I'm away from home.
One area I wish they did have roaming for is in Canada. Have two upcoming business trips there. Zero feedback when I asked customer service on Canadian roaming with my initial domestic roaming request(but they did set up domestic roaming and send me initialization directions), and to another email specifically on Canadian roaming when I asked last week. Most likely fallback will be a SkyRoam mifi device en route to me and a Piranha Mobile roaming sim chip I have in an unlocked ZTE/T-Mo flip phone. Could use my CellNUVO phone in airplane mode so it doesn't use cellular data/voice/SMS but gets wifi signals to use apps including email/translator/gps maps...but don't want to get hit with international roaming charges if airplane mode doesn't work properly.