volte有効化ぐぐったら出てきたけど？

Go to Settings > Call Settings (Sim and Network)
Tap Cellular networks > Enable Voice over LTE On/ Off.
Alternatively, you can follow a much easier way:

You can simply dial *#*#4636#*#*
Choose Phone information.
Click " Phone Information" and Toggle VOLTE Provisioned.
Restart the phone and you are done.