Suddenly your cell phone shows “Car Kit” and seems to work except you can't hear people and they can't hear you – just a strange noise.
Do you have a baby in the room? On some cell phones, if you get the charger slot wet (or a baby sticks his/her tongue in it) it can short out the phone and make it think a special “car kit” device is plugged into it (whatever that is). You look on all the settings and there is nothing to adjust anything like Car Kit.
To repair this – make sure the charger slot has dried out. Then, wiggle a piece of metal in the slot until “Car Kit” disappears (bascially you are toggling the Car Kit setting back off). The end of a butter knife, a paperclip, a piece of tin foil, ... something like that does the trick.
And now, keep the phone away from babies.