You have to also take into account people's time lost and inconvenienced in scenarios where SMS fails. For example, if the user doesn't have their phone on them

You have to also take into account people's time lost and inconvenienced in scenarios where SMS fails. For example, if the user doesn't have their phone on them but needs the money, then what? Or when the SMS message doesn't get through for whatever reason. Or where there is a no signal.