Everyone has a unique phone number. Are you looking to have them sign up multiple times with the same number? You can also require an email address as well if

Everyone has a unique phone number. Are you looking to have them sign up multiple times with the same number? You can also require an email address as well if you prefer. It's definitely going overboard, but I've personally never experienced bots really signing up for anything I've made. Sounds like you have a great product that bots can take advantage of if you have that many signing up.