Cell phone coverage is spotty in many areas -- to the point that it depends on how far you are from a freezer or light fixture or whether the building has a steel frame (Faraday box) or which side of the driveway is in line with the cell tower. I think that, even at opt-in, requiring a cell phone contact in order to log in is a disaster in the making, and I do not want it. If a two-stage verification is needed, do something else that does not involve cell phones.