Android phone comes with an inbuilt functionality of auto rejecting a call without even ringing your phone! It is so simple and comes with wide range of match criteria for incoming phone call. Now it is the answer for how to set your android phone on auto call reject mode.
How to do it? Just go to Settings –> Call –> Call rejection. Once you are at this step, first you can define auto reject mode. You can set it to off, All numbers or Only black list. I am not sure why one wants to block all numbers but still it might be helpful if you are in some important meeting or just want to take rest without any disturbance.
The next option is Black list, which allows you to manage numbers in the black list with the option of match criteria to block certain pattern of phone numbers. There are 4 types of match criteria using which you can black list any kind of number you want.
This is really helpful to get rid of some annoying marketing calls. Also you can block all calls from some country or states as well!