iPhone doesn't support this.
I used a belkin Bluetooth audio adapter (Bluetooth in, headphone jack out), connected to AUX
This worked fine.
iPhone knows, that there is no microphone on the belkin device, so it keeps the calls on its speakers or on my Bluetooth headset (plantronics voyager)
set your iPhone 6 to put phone calls to iPhone or speaker rather then karma. the music will play fine and calls will go to the phone. if you want to have a Bluetooth head set with the audio on the karma, you'll need the Belkin (or like kind) adapter per the other responses.