There are proactive and reactive methods for dealing
with these obnoxious robocalls. On your own device
you can take proactive measures to at least cut down on the number
of robocalls you receive by manually preventing repeat offenders
from calling your number and hiding numbers that aren't on your contact list.
Tap the "call history" tab in the Phone app after launching it.
In the pop-up menu that opens, select "block" after tapping on the number.