Why can I not hear when I make phone calls on my iphone 4s?

Whenever I try to phone people, I can't hear either them speaking or the dialing then beeping. It has worked in the past and I have not changed my settings, but the last call which I could hear I listened using my headphones.

On these devices, when you plug in the headphones, the general volume for the phone (calls/dials, etc) is automatically adjusted, it will mostly turn down your volume. Once you unplug them, it should set your volume back to where you had it prior to plugging the headphones in or it simply lowers the volume completely. This happens often, it's kind of a "glitch". If this isn't what's causing the problem, then its a hardware problem and the phone needs to be checked.

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Easy fix. When you make another phone call, just before you put it to your ear or on speaker, turn up your volume like you would when you want to listen to something a bit louder.