Doable with Cat-M1, which supports VoLTE, has low cost modems with extremely low power consumption, and as a bonus has a 16-20dB link budget advantage over broa

Doable with Cat-M1, which supports VoLTE, has low cost modems with extremely low power consumption, and as a bonus has a 16-20dB link budget advantage over broadband LTE, so it'll work in many places where conventional handsets are unusable. The problem is the extremely limited market for this sort of phone (and patchy support by network operators), rather than any real technical constraints.