Comment by LouisSayers

SMS for any significant action on an account is terrible.

  1) Phones can be lost or stolen
  2) People move country
  3) SMS attacks
  4) Phone numbers get reused
  5) Users must maintain a paid phone plan

For the love of science, DO NOT tie accounts to phone numbers!!!

-- edit --

I updated the first line to clarify that I'm not talking about one-off notifications etc.