There are also assumptions that everyone with a mobile phone will have the same phone number for a long time, never have their phone stolen or destroyed, could easily replace it and restore from a backup if it were, and pay their phone bill in a way that never results in cancellation (and thus forfeiting a phone number).

I know someone who works at a public library and regularly interacts with folks who've found hard times. From the stories I've heard, it's a fairly common situation for someone to want to reset their password, then be told by the site that they will receive a text message on a phone number "ending in ####". This is a challenge for those who had to get a new phone number since they signed up.