The imp handles calls in 3 different ways - the Greenlist, the Redlist, and the Greylist.
The Greenlist is a list of contacts you want to hear from. These are approved callers and can ring your landline phone.
The Redlist is the antithesis of the Greenlist. The Redlist are callers that you have identified as unwanted. The imp will simply hang up on a redlist caller and your landline will not ring.
The Greylist is everyone else. These can be unknown callers. These can also be known callers, but ones that you don't want to be bothered by. The Greylist does not let your landline ring, but the imp allows them to leave a message that is delivered to the imp app on your smartphone.