Suppose for some reason you lose your phone number or retire it for something local. Then suppose it is recycled to people with bad intent. Could the number be used to fetch passwords? Could they get into your e-mail?
You bet they could!
Millions of people get new phone numbers, some of which have been used in the past by someone else. In the best case, the new owner gets annoying FaceTime requests from teenagers. In the worst case, they get texts every time the previous owner makes a bank deposit. Why? That someone hasn't changed his phone number with their bank.
Many companies have built their user authentication programs around phone numbers.
One person who tested the security of phone numbers found this out. He got a new number then went to a large e-mail provider and typed the new number in the login. The provider then offered to reset his password and sent a SMS message with a code. He got in using the code. Except for one thing: He got in to another persons e-mail, the one who previously owned the phone number.
Our phone numbers are tied to our identity.
It's not just recycled phone numbers that can be vulnerable either, since there are hacks that allow the theft of numbers. That makes everyone with a phone vulnerable, but there are some things you can do:
1. Close accounts you don't use.
2. If you don't have to give your phone number online, don't.
3. If your number has changed, go to every account and update your phone number.
4. You can also disable SMS text codes.