This might also be a Bluetooth issue. I noticed in my 2010 BMW that the Bluetooth call quality has been terrible as well on my iPhone 6. I never had a problem with all of the previous generation models. I did see some other threads about Bluetooth audio issues on the 6 in various cars including BMW. Unfortunately, I cannot upgrade the firmware easily in my car since it's an older model.
Earlier I was on the phone with my wife over Bluetooth and VolLTE and the quality was awful. I changed back to CMDA and called her back and it was much improved. Just now, I made a call on the regular handset over VoLTE and it was fine, and then I tried over Bluetooth and it was full of static.