[abbe_k]

I'm pretty sure that most operators either use analogue or digital end to end, no mix.

VoLTE (Voice over LTE) have some other advantages over other calling applications. VoLTE is most likely on a very high QCI (priority), which means it probably has less delay and less packet loss than other traffic. VoLTE also often uses header compression over the air which means the packets gets a lot smaller and therefore also reduces packet loss. I don't think VoLTE usually have very high bitrate though, maybe around 10kbit/s. This is one area where other apps could be a lot better.

EDIT: I am of course only speaking about mobile calls. For landlines I agree that other applications provide better quality.

[Rjevski]

The issue is that VoLTE only works for calls made to the same carrier's subscribers, as soon as you call someone on another carrier it will go through a shitty interconnect with a bad codec (usually a variant of G711 which is a joke in 2017).