Motorola Moto G 4G — trouble answering calls

when my phone rings if it is on my home screen ,I just swipe it, some times it works ,some times not.
but sometimes it shows a circle of rings going outward, it shows a phone ,I have tried tapping the phone nothing, I have tried swiping ,nothing ,and anything else I can think of ,nothing happens .
it just goes to voice mail. what am I doing wrong.

 please help this is a new phone. I am desperate to get it to work.