If you want to know the real reason for this, it&#x27;s because normal cell calls still have to go over POTS (<a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Plain_old_telephone_service" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Plain_old_telephone_service</a>) on the backend, which typically uses 8-bit mu-law companding of audio at 8kHz.  This compresses the typical frequency range for phone calls to 300-3000Hz, which is a lot smaller than the real range of human hearing, although it covers the range used by human speech so it was considered totally acceptable <i>before</i> unreliable wireless packeting became the typical last leg.<p>VoIP, FaceTime, Skype and other IP-only technologies can use much better audio codecs, going all the way up to CD quality audio (44kHz, 16bit). This makes a huge difference when there is background noise and clipping and less than optimal transmission conditions.<p>(Source: I used to work for an IVR company.)