Electronics Repair Questions? Ask an Electronics Technician
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It sounds as if you might have a problem with either the microphone in the iPhone 5, or perhaps hopefully the microphone volume setting.
Go into the Volume settings, and make sure the microphone is turned up almost all the way. Try voice dialing, and calling again, and see if people can hear you when you shout, at least. If they cannot, and if voice dialing still does not work, you may well have a defective microphone in your iPhone.
That could well be, if you are doing voice audio through the Pioneer device.
Does the iPhone work OK just as and by itself? then its microphone is not at fault.