I found this out accdentally, but using a regular stereo cable from my DI to the car's aux input, I was able to hear calls through the speaker, and they could hear me via the phone's built in Mic. I was surprised the first time I heard it ring, but it worked very nice, no issues that they couldn't hear me and the sound quality was top notch. I really like how it will mute whatever else is playing in the meantime, like Slacker / Pandora, and continue once the call is over.
Hope this helps.