It’s easy to forget that our pocket computers are also telephones, and thus emergency calling devices. These are regulated with good reason. The operator/their partners have to test the device on their network to ensure it is compliant and emergency calls can be made as expected; they also need to build the VoLTE/VoWiFi/IMS settings for that specific network into the handset’s software before it will work - VoLTE has many complications, it is not one size fits all. Accordingly, some operators allow BYOD, while others will only whitelist the specific hardware and software combination they have tested and signed off on.
Over 2G and 3G, voice calls are circuit switched. VoLTE and VoNR are packet switched, over IP, VoIP. Totally different. VoLTE is not as standardised as it may seem from the outside whereas 2G and 3G voice calls were.
Internet access is not regulated as an emergency service.