There is a switch thing on the side of the phone, when you switch that over to the right, the orange will be covered up and your phone will be on ring. However, if in the phone settings you have the phone on vibrate, or the volume down, or on do not disturb then the phone will not make sounds. Go into Settings -> Notifications -> Phone -> Sounds to change the sound or
Settings -> Sounds to set a text tone or ringtone or you can turn up ringer volume which could be your problem
or go to Settings -> Do Not Disturb and turn off Manual to receive calls and alerts (you can also do that in control center by turning off the moon). ^One of those should fix your problem.