It's the phone. I'm on my second ONE X+ and both have bluetooth call quality issues. Music streaming is fine via BT.
The first phone was two-way - i.e., both me and the other person sounded distorted
The replacement is different in that the other person sounds distorted to me, but not v.v. I tried on 3 different BT systems (two ear piece, one car), identical for all three.
I have a case in with HTC, who acknowledged the issue but could not provide a timeline for a fix.