Facebook, Google, etc. ask your phone number for 2 factor authentication
This is a trick to get your phone number
According to US National Institute of Standards and Technology (NIST) - is insecure and should be banned.
A hacker can easily intercept your sms and hijack your account.
Use email instead, it is more secure
Unfortunately this is true about banks also but a lot of banks also ask for a code that is printed on your debit card.