I found a notice of missed call on my smartphone. It's strange, because I carry it with me all the time no way I missed call. Only one possibility would be this, call tone volume had setting as vibration. I check a volume of call tone and found It was setting as vibrate. I didn't set it that way. Then, I found out what happened. My smartphone has volume button on the side body of phone. Every time I hold the phone I touch the button unconsciously and lower volume until it become vibration set up.