Then how do you turn it off? There is no column to turn off HD calls in the settings, and Unicom card can only be turned off without a mobile card.

Unicom users can send TDVoLTE to 100 10, and reply y to confirm that the VoLTE function is turned off. System reply: Dear users, your VoLTE function has been turned off, which will take effect immediately. Thank you for your support to China Unicom. Effective immediately. Failure to reply means that the application has been cancelled.

You can turn off the VoLTE function on your phone in the following ways:

Huawei mobile phone: settings → wireless and network → mobile network →VoLTE call;

Xiaomi /OPPO/VIVO mobile phone: Settings → Dual card and mobile network → Click SIM card information and settings (Unicom) →VoLTE HD call;

Samsung mobile phone: Settings → Connection → Mobile Network →VoLTE HD calling SIM1;

Meizu mobile phone: Settings → operator network → Click on SIM card management (Unicom) →VoLTE HD calling.