I have an iphone 5s and I use the navigation system on the phone for directions. I plug my phone into the usb so it can charge while the phone is on navigation. I can't hear the voice prompts at all. I've turned off bluetooth, but still no voice prompts from the phone. When I power down the radio/sync, I can hear the voice prompts. I have Sync, no MFT. Any ideas on how to get the voice prompts to work? Thanks.