Business Insider reports that the feature will filter out any incoming calls that aren’t from a number in the Contacts, Mail or Messages app on the device. These calls will be automatically redirected to voicemail and the phone won’t ring.
The feature will be disabled by default when it does arrive, meaning users will have to go into their phone’s settings in order to turn it on. Additionally, it can only look for numbers that aren’t saved in Contacts but could be non-robo callers in Apple apps, so if a phone number was mentioned in a third-party app like Facebook, it will likely be identified as spam.