VoLTE has gained the option of using a higher bitrate codec than has been in use since the early GSM days. And the GSM bitrate and codec was chosen to pack as many voice calls as possible into the bandwidth of the GSM protocol.
That said, if it is iPhone to iPhone, it may well be that the call never touch the carrier systems and instead happens over iMessage for all i know.
> That said, if it is iPhone to iPhone, it may well be that the call never touch the carrier systems and instead happens over iMessage for all i know.
That would only happen if you are using Facetime Audio
When calling on my iPhone the call quality is significantly higher in many cases. I'm not sure if it requires both callers to be on an iPhone or on the same carrier or what. But sometimes the calls are as good as the audio coming from the Music app. Other times, they sound like StarTAC calls. But something has definitely changed in the last few years for certain types of calls. (And I've found FaceTime audio calls are always high quality audio.)