That would be a factor you as the user would have to take into consideration when deciding whether to opt-in. I don't think it should be a big consideration in deciding whether to implement the scheme; I would guess that the vast majority of users, a vast majority of the time, have good cell coverage when/where they are logging in. For those that do not, don't opt in. Simple.

On the other hand see my comment below. I don't think SMS is the right way to go, and an RSA-like solution would work with your smartphone but not require actual coverage at the time of use.