Sometimes your iPhone enters a mode where you can't answer it unless you use your earbuds or bluetooth headset. Well, you CAN answer it, but you can't hear anything. The handset speaker stops working. It appears that the iPhone still thinks the earbuds are plugged in so it turns off the speaker. This is how to fix this problem without having to go to the Apple store.
NOTE: You could probably use alcohol to do this as well. The idea is to JUST GET THE TIP DAMP. Don't flood it. There is a moisture sensor down there that you don't want to trip.