If you've had an iPhone for some length of time chances are you have experienced this symptom. You make or receive a call but you can't hear or talk through the normal mic and earpiece.
It's happened to me twice now. The first time I finally figured out how to fix it. It was a simple 32 step process that apparently was no more effective than a rain-dance.
The real reason this happens is that dust or pocket fuzz or some other foreign material gets in to the earbud socket. This triggers earbud mode without the benefits provided by the earbuds. Eureka!
Clean out that socket and you're golden.