Call quality is bad with Calling PLUS. I had to turn it off.I tried this (I'm on Sprint, too) but found that call quality took a major hit. I'm sure WiFi speeds have an impact, so YRMV.
My Call quality hasn't changed at all. Never had a problem with mine. I think it depends on where you live. I keep calling plus on even though I don't utilize it as much. Still so used to not being able to use data while on callCall quality is bad with Calling PLUS. I had to turn it off.