a telephone with a call screening device is able to screen automated calls by transmitting a message containing an instruction upon detecting an incoming call . the instruction from the call screening device requests that the calling party respond to the message correctly and in time before the call screening device allows the telephone to ring and the call answered . the instruction contains , inter alia , a string of randomly generated numbers that the calling party is asked to dial as part of his / her response . in the event that the response is not received in time or the response is incorrect , the call screening device terminates the call without the phone ever ringing .