As far as my knowledge goes...
VoNR stands for Voice over New Radio, which is a voice-calling technology that operates over 5G networks. The latest iPhone models, including the iPhone 12 series, support 5G connectivity. However, the specific availability of VoNR on iPhones may depend on various factors, such as carrier support, network infrastructure, and software updates.
VoNR allows for high-quality voice calls over 5G networks, providing improved call quality, faster call setup times, and better overall voice communication performance compared to previous generations of mobile networks. It utilizes the advanced capabilities of 5G technology to enhance the voice-calling experience.
To determine if your iPhone supports VoNR, you can check with your mobile carrier or refer to the technical specifications provided by Apple for your specific iPhone model. Additionally, staying up to date with the latest iOS software updates can ensure that your iPhone has the latest features and capabilities, including any enhancements related to voice calling over 5G networks.

Please check the bandwidth that you are getting. Disable WiFi before running this Field Test.
You may check the current LTE band/5G/4G/3G by dialing *3001#12345#* and pressing the call button and then selecting the Serving cell info option
Bandwidth on the 2nd and 3rd Screenshots.
See the pic below which is self-explanatory


E, 2G, 3G, 4G, LTE, VoLTE, 5G all are Carrier Specific features. Contact your Carrier (Cellular Service Provider), the agency that issued the SIM / eSIM to you. About the LTE options on your iPhone - Apple Support (IN)
See the Pic below and explanations thereof


The pic below is self-explanatory


However iPhone model specific feature is listed here --> Identify your iPhone model - Apple Support . and tap on "See the Technical Specs for Model XXXXXX" under each model and Scroll down to "Cellular and Wireless" section.

Know it here --> All about G, E, 3G, H, H+, 4G, VoLTE, 5GE and 5G symbols ...
