My main question is will it work with MVNOs, do they need to implement it or does the parent network handle it for the MVNO?
I’m guessing it could cause issues with virtual numbers on Sim cards.

Some docs.

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Sounds like NV will be all up in your business…

One downside I can see is that this forces you to use the service you’re logging into on the phone itself and excludes those who don’t have a compatible phone. SMS 2FA on the other hand is independent of the device and you can even use it with a flip phone. If you’re going to redo 2FA in a non backwards-compatible way why not do it properly?

Furthermore this won’t protect against fraudulent SIM swaps which is the main threat when it comes to SMS 2FA.

That’s not necessarily a downside. It’s a convenience for when you happen to be accessing the service from the phone – no point sending an SMS and having you type it in (although modern OSes recognise and simplify this by suggesting the received code on the Keyboard).

If you’re on the phone, then the authentication is transparent, which is convenient. If you’re on a different device, then you can still use the good old SMS or input a code etc.