I picked up a Space Gray iPhone SE 64 when the Apple Store opened on 3/31. I was really looking forward to getting back to a pocketable iPhone form factor from my iPhone 6.
I am on the phone all day for business. I have several over-the-ear boom mike Bluetooth headsets that I have used for years with various iPhones. The sound quality has always been excellent.
Bluetooth headset telephone audio quality from the SE is distorted to the point of being unusable on all of my headsets. The proof (to me, at least) is that I can unpair them and pair them back to the iPhone 6 and make FaceTime audio calls without a problem.
After some experimentation with various callers, it appears that this is a reception-only problem from the SE to the Bluetooth device.
Interesting. I've had no Bluetooth problems in my car.