This was driving me nuts, as well (hands-free/BMW), and I can confirm that turning off wi-fi fixed it for me. For me, it even works
after a call is already in progress.
Although I'm glad to have a workaround, I hope this issue is identified and quickly squashed. I'm often in and out of my car several times a day, and it's getting really old, having to futz with wi-fi every time. I also keep forgetting to turn it back on, once I'm at home.