HMD is fixing the VoLTE issues with the Nokia 7 plus
The June update seems to have broken the service on already supported carriers like Jio and Airtel. Other carriers are still working out how to enable the service after the Nokia 7 plus was updated. The good news is that people are now reporting success in using VoLTE.
The update also messed things up for people running the Android P Developer Preview and required them to restore the software back to Oreo to make the phone usable again.
News credit : Gsmarena