Make sure the phone is not on vibrate by pressing and holding the # button for a couple seconds. There will be a vibrate symbol that appears on the top right hand of the screen when the phone is on vibrate only.
Turn up the ringer volume by going to Settings> Sound (6)> Ring Volume, then toggle the volume higher. The bar will display blue to demonstrate how high the volume is.
If the problem persists after following the troubleshooting solutions and you believe your phone is defective, please fill out the replacement request form.