Harassment (Silent Calls)

This number have been calling me, you'll answer then the person will go quiet but you can hear background sounds. I called back in the first time and it was the mother who picked asking me who am I when I said I got a call from this number so it's clearly someone going around her. I didn't want to block the number so the person won't call with a different unknown number but now I have no choice but to block it. If anyone close to her sees this post,please alert her on what the person have been doing with her number