SIM Swapping
Good in depth blog over on Krebs on Security on this. Seems that "SIM Swapping" is a thing - basically persuade/bribe some mobile phone support/sales body to point a mobile number to a new SIM (As you would if you lost your phone, switched provider etc).
So if you're "forgot password/2FA" process involves reset via SMS... Social Engineering to get e-mail/twitter handle and mobile and SIM Swap and ...