Have you shared your cell phone number on any web sites? Once it’s publicly available, your cell phone number is no longer secure. So you need to stop using SMS authentication for your logins. Instead, consider using an authenticator app like Google Authenticator or (our preference) Authy.
This NY Times article explains why your cell number is not secure. It illustrates how your cell phone number can be used by hackers to break into your online accounts and steal your data, or even your money.