I think it was AT&T that had a test number you could call to hear a higher quality phone call (I was pretty young at the time, so my memory is fuzzy). I remembe

I think it was AT&T that had a test number you could call to hear a higher quality phone call (I was pretty young at the time, so my memory is fuzzy). I remember it sounding very good, but that test number was the only time I remember hearing that quality over the POTS. The VoLTE and HD voice I occasionally get on my iPhone reminds me of that system.