I recently had a problem with my iPhone where I couldn't hear any calls. I'd put the phone up to my ear and people could hear me, but I couldn't hear them. If I put the iPhone on speaker mode, the external speakers worked fine and I could hear the call. I just couldn't hear when it was up against my ear.

The fix I found was to use the headphone jack. Just get some headphones or an earpiece with the correct jack and plug it into the jack on your iPhone and then unplug. Do this a couple times, and ta-da, the speaker on the iPhone starts working again and you can hear your calls.