Add-on Phone Verification For Upgrades (or Registration)

[Mouth]

I'm not sure their model really fits this
It seems they require phone authentication every login ...

It's a REST API, so you just utilise it where-ever you like. No need to configure it on logins at all, but perhaps just on certain usergroups? This way a user (or admin) can decide if the want the 2-factor active for their account. And you can utilise it for upgrade/payments etc.