Exclusively? You make it sound like VoLTE is mandatory. That is not the case, to my knowledge. On a 4G network, for example, one does not always have VoLTE available, and yet one is always able to place voice calls. Since your conviction is palpable, if you could please provide a reference then that would help further the discussion. If not, then no worries, will find the information on my own.
> 4G and Evolved Packet Core (EPC) architectures do not include support for circuit-switched voice and video calls. Two tracks are available that provide interoperable voice services on 4G smartphones: Circuit-Switched Fallback (CSFB) and VoLTE.
Circuit switched fallback means falling back to 3G or 2G (or maybe CDMA) for voice calls; if such networks are no longer available in a given area, VoLTE is indeed mandatory if voice calls are to be supported. (Which is often mandated by regulations, so networks sometimes even block non-VoLTE UEs from attaching.)
