My apologies for misunderstanding. I had just answered another poster who was trying to use a Bluetooth Speaker for sound, but the microphone of the call to speak. Which won't work unless the speaker has a microphone.
So to be clear, you're saying if you use your iPhone for a hands free call, the person you're speaking with can't hear you, correct? One thing to try is to make the handsfree call and then use the side volume up button, which may be too low for the microphone. Give that a try and report back and if it doesn't help.