Apple has improved the security of FaceTime and iMessage, its
voice/video and multimedia chat communication tools. The services
received two-factor authentication today as an option for users to
enable, meaning that even if someone uses their Apple ID email and
password to enable iMessage or FaceTime on a new device, they’ll still
need to use a PIN from an existing trusted device to gain access to
those services.You may recognize the system from iCloud’s two-factor authentication, or if you’ve tried to set up Keychain to keep your passwords in sync between Apple devices. If you’ve previously enabled two-factor for iCloud, it’ll also be enabled to FaceTime and iMessage. The additional level of protection applied to these services helps ensure that people will have a harder time grabbing potentially private images from your iMessage history, or pretending to be you via online communication methods.
Two-step comes into play when users log out of an account on their device and try to log back in, as well, meaning you’ll have to get that trusted device out should you temporarily disable your account on the device, or in some cases if you run a system update or switch SIMs.