Nomorobo uses a feature known as "Simultaneous Ring". When simultaneous ring is enabled, your phone will ring on more than one number at the same time. The first device to pick it up gets the call and the other phones stop ringing.
So, when the Nomorobo number is enabled as a simultaneous ring number it is the first number to screen the call. If it’s a legitimate call, the call goes through to your number. If the call is an illegal robocaller, Nomorobo intercepts the call and hangs up for you. Your phone will ring once letting you know that the robocall has been answered and stopped.