It is mostly in the power of the Clever Dialer users to classify a phone number as spam. The users can rate the seriousness of a phone number on a scale from one star to five stars. One star signifies that the phone number is with a high probability untrustworthy. Five stars signify high trustworthiness. Whether Clever Dialer issues a warning against a phone number depends on various signals. The most significant signal is the number of users who rated a number as spam.
Positive ratings can compensate negative assessments. This is important to prevent the abuse of the rating system.