If you have a jailbroken iPhone, you can do 2-way voice chat over BlueTooth. iBlueTooth Call is a cydia app that allows you to do two-way voice chat with your friends and dear ones via BlueTooth.
iBlueTooth Call supports iOS 3.0 and above firmware and requires a built-in mic and BlueTooth.
For iPod Touch 2G and below, you will requires a headset with a mic connected, so that you can use this app properly.
You can download this app from BigBoss Cydia repo.