IPhone :: Call Volume - Impossible To Hear The Other Person

This is my first iPhone, and I don't usually make or receive a lot of calls, (I communicate much more with texts.) I received 2 calls recently and it was almost impossible to hear the other person. I tried hitting the + on the side, but it didn't seem to affect the sound volume. Is there something I am missing in the settings, or something else I am missing? I don't use any type of headset/headphones, as those are prohibited at work. I will say I have a Belkin case on the phone, so it's possible I may not have been hitting the volume controls like I thought. I just wondered if there was something I was missing elsewhere. I am hoping this is just operator error and nothing wrong with the phone.