Thank you for the condescending explanation. I understand how two factor authentication works, I was specifically rebutting your assertion that people would be able to make anything other than a guess as to what sort of cell coverage they can expect to encounter at exactly the times when such a scheme is at issue.

Your presumption that the "vast majority of users, a vast majority of the time" would have good cell coverage from anywhere that they might log in betrays a complete disconnect from the realities of exurban America, let alone anywhere else.