Facetime Audio is so crystal clear for me that it feels like I am sitting next to my friend/family vs analog lines where there is generally an echo, and other weird delays. When I talk to my grandparents it always feels like we need to put pauses in our conversation to make sure we don't start talking at the same time.

doesn't verzion use VoLTE? which would mean that your normal phone app isn't your "analog" phone anymore.. (https://en.wikipedia.org/wiki/Voice_over_LTE)

Basically there aren't many phone calls over non LTE based services. Especially since VoLTE can be used over WLAN via Wlan calling techniques. (the latter isn't supported in all networks.)

VoLTE is only between your phone and your carrier; if you're calling someone on another carrier as far as I know it still goes over legacy garbage with a shitty codec (or worse, an analog link, with in-band signaling).