Sometimes when you unplug a headset from the iPhone it gets lost and confused and you can neither hear nor talk using the internal speaker or microphone. Interestingly - it works as a speakerphone but not as a regular phone.
Quick tip: If you answer a call and put it to your ear and cannot hear - don't panic - switch to speakerphone this will work.
Solving the problem: Solving the problem is simple - just insert and remove your headphones three times and voila - it works!
References:
http://discussions.apple.com/thread.jspa?messageID=6830349
Posted by csev at March 14, 2008 06:04 PM