Two-step verification by mobile phone. Quick and easy. All the user has to do is add their mobile phone number to their account - they then have to verify it and they can use the two-step verification.
As there are a large number of SMS providers, it is possible to add further providers to this add-on with little programming effort.
A provider is available for this add-on. I provide another one via add-on.
Others are conceivable.
Everything you need to know about this can be found in the plug-in for this add-on (Twilio).