It is ridiculous to think that your number is private and even moreso to think that someone can steal money with just your cellphone number.
Of course you could be phished or tricked by SMS but to expect your number to be private is to expect everyone ever who you give the number to go to extreme to keep it private as well.
If you ever gave your number to someone who downloaded an app which has permission to contacts your number is no longer private (Facebook has taken big advantage of this, I'm sure there are less than reliable apps that took bigger advantage).
It's actually fairly easy in some cases. If the user is with Virgin mobile, their 6 digit password can be bruteforced in a couple of minutes, and then every text and call they've ever made is public. I've made a fuss to them about this quite a few times, but they've never done anything to fix it.
Of course you could be phished or tricked by SMS but to expect your number to be private is to expect everyone ever who you give the number to go to extreme to keep it private as well.
If you ever gave your number to someone who downloaded an app which has permission to contacts your number is no longer private (Facebook has taken big advantage of this, I'm sure there are less than reliable apps that took bigger advantage).