SIM swapping
In SIM swapping an adversary steals your number and assigns it to a new SIM card in a phone they control. It is the same process you go through when you get a new phone and the mobile carrier gives you a new SIM card.
The adversary can now use your SIM card to steal further information to log in to your accounts and either enter a verification code or reset the account password using the code or link sent to the phone.
Prevention
Contact the mobile phone operator and add extra security or temporarily freeze number porting to help protect yourself from SIM swapping.
Check if your accounts let you use a non-SMS multifactor authentication option, in which you provide two pieces of proof to verify your identity.