CarPlay - Incoming Call issues

I have a Kenwood DMX7704S car stereo and am running firmware 1.4.0008.1600. When I receive a call on my iPhone 6 (iOS 11.0.3) while listening to USB audio on the car stereo, the CarPlay incoming call screen appears and the audio mutes, but when i push the "answer" call button, I cannot hear the caller and they cannot hear me. When I am listening to music using the CarPlay Music app, incoming calls can be answered and everything works normally. Please help.