HEY YEAH!! THIS IS HELPFUL TO ME!!
I got my solution on this problem from RYAN HEALEY's post: All I went through of fixing it, all the possibilities that causes why there is no ringtone sound when somebody is calling me while all the rest like music, alarm,games, etc are working fine. After reading the idea from Ryan Healey, I checked my phone to find where is the switch can be located & I found the switch on the left side of my phone. The button switch was pressed down so no matter what I do, there really NO SOUND at all.
You need to MAKE SURE the SWITCH on the left side of your phone (Iphone 5S) is ALWAYS TURNED UP. After this you can expect the sound to the FULLEST!!
Thank you RYAN HEALEY, you gave me the idea!!