After the June update, many users of the Nokia 7 plus reported having issues with VoLTE.
Juho Sarvikas, CPO of HMD Global, tweeted that the fix for VoLTE problem is out.
Modem config file wasn’t loading correctly. The update should be available over the air now, and you should be able to use VoLTE without any problem. If your network support VoLTE, give it a try and report back if you experience any problem.