As of the end of March 2022, devices that do not support the au high sound quality voice call service "au VoLTE" can no longer be used. Customers using devices that do not support "au VoLTE" may continue to use their devices until the discontinuation. However, after the service discontinuation of these devices, a switch to devices that support "au VoLTE" will need to be made to continue using au services.
If you are using iPhone 7, iPhone 7 Plus, iPhone SE, iPhone 6s, iPhone 6s Plus, iPhone 6, iPhone 6 Plus, or GRATINA 4G KYF34 with the settings for the "Voice & Data" communications options set to "3G" or "4G and VoLTE Off", communications will become unavailable once this service ends. Please set your usage settings to au VoLTE. For details, check the following.