My phone calls are exactly the same as they were with the regular S4 -- very good. I have had a number of calls and always check new phones by leaving voice messages for myself with the phone. It sounded clear and almost like a landline. On my end calls are very clear and I had to turn down the volume from max because it was too loud. Your test model must have had a problem.
I had problems with my mic, earpiece and speaker. Voice dictation wasn't working and people were complaining that I sounded robotic. I then realized that there was water stuck in the microphone and earpiece from when I had it in the bath. Simple fix, ... (continues)