The port-in process, to Mint Mobile, could not be any easier or faster.
I followed the activation steps, which were clear and concise, and my number ported from Sprint (free year) almost instantly. (Within a minute or two.)
THE ONLY THING that tripped me up was that I had Google Voice set to use my GV number when I made outgoing calls-- and Google Voice didn't recognize my new Mint Mobile service-- so I would initiate calls and they'd terminate before being completed, with no error messages.
It wasn't Mint's fault nor Google's. I had simply forgotten that I'd set Google Voice to use my GV number. Everything worked exactly as it should have.
As soon as I figured out what I'd forgotten to change, I was off to the races.
So if you port to another provider and have trouble making an outgoing call-- it could be your Google Voice settings!
While lexusl21 is correct that GV needs data for full operation, it's possible to call via Google Voice without any data connection under specific circumstances, at least on a carrier that does not require VoLTE.
For it to work without data, the outgoing call must be to a number that you've either called or received a text from previously via GV. Google Voice uses a system of creating a second imaginary phone number for each contact you want to interact with-- if that 'spoofed' number already exists, stored in the local GV app, on the phone you're calling from, you can call to that number without data access.
I just confirmed this by turning off WiFi and cellular data on my phone, and using Google Voice to place an outgoing call to my home phone.
Without data, you will not be able to place a call to a number you've never called before (or received a sms from) via GV.
TLDR: There are some caveats: I suspect that specific circumstances may cause some outgoing calls to fail via GV-- in which case you'll have to call via the native number. I suspect, but have not confirmed, that the local GV app must have synced data with GV after the creation of the spoofed number for the contact you're wishing to call.
Although you didn't ask about texting..... GV sms requires data. If you have GV set up to forward sms to the native messaging app, you would still receive incoming sms sent to your GV #, via the native messaging app. You would only be able to send a text via the native phone # and native messaging app, not via GV sms.