The problem with this is some phones have crappy connections with the OEM BT stuff.
My HTC EVO is less than a year old. My M is a 2007. People can hear me, but it’s not as good as you’d think. Other phones though work fine in my car and I’m sure my phone would work fine in a newer car.
Handsfree BT stuff is nice but it isn’t the cure-all for taking calls in the car.