Because cellular standards up to 3G were designed to support circuit-switched voice lines, i.e. you would get a dedicated slice of network resources for your ca

Because cellular standards up to 3G were designed to support circuit-switched voice lines, i.e. you would get a dedicated slice of network resources for your call. That meant the voice interface had to be built into the base standard just like data communications. 4G and 5G treat voice traffic as packet data (not quite like other data, I believe it gets slightly higher priority) so VoLTE/VoNR is just running a data application on top of the 4G/5G data link. That means you can specify the voice connection seperately and many carriers take advantage of that to do things their own way (to the detriment of consumers).