I don't know that I've ever had to indicate a carrier when signing up for text messages. Parents, from the reaction we've had, seem to have had the same life experience. The list is also confusing (There's no AT&T? Apparently, Verizon will soon be several different options?). We've had to tell those parents a few times to just try the different ones until you get a message. I've also had to tell a couple to "contact their carrier" if they don't get a message at all. They don't want to hear that. I don't want to say it.

Apparently, the whole problem stems from the fact the back-end is really just sending emails based on the format provided by each carrier, not sending actual texts. It seems like not using this type of workaround would also allow you to offer a feature like dedicated text messaging in the event of an emergencies. ¯\_(ツ)_/¯