The most annoying thing about having a non-contract phone...
is that without the carrier bloatware, you often don't get access to the better features (e.g. VoLTE, VoWiFi (wifi calling) without seperate apps that may or may not work, access to certain LTE bands (Band 20 800MHz, I'm looking at you Three). And worse, if you've got a phone they don't officially support, you're not going to get them either. I've got a OnePlus 3 that I'm still really happy with, but I'm basically told I'm SOL when it comes to VoLTE. It's supposed to work via their app, but naturally doesn't.
Course, you take the carrier crap, and you end up with lagging updates, apps and services you don't want ("value added"), if it's an option for your phone.
IIRC Three were using "4G Super Voice" as a marketing tool/gimmick that they were using to sell more handsets. I'm hoping the shift towards SIM only will push towards having some sort of standard operating setup with VoLTE/VoWiFi. And maybe all the networks could try testing the generic firmware models on their network and enable the features if they're supported. In their defense, apparently EE do support VoLTE and VoWiFi on the OP3, but I'm not willing to pay a 50-100% premium on my SIM only contract in order to access them.