Your device is connected to the 4G mobile network when in standby mode, but it switches to a 2G/3G network when making a call. After the call has ended, it will switch back to the 4G network.

If you want to make calls using 4G, first contact your carrier and activate the VoLTE service for your SIM card.

Then, on your device, swipe down from the status bar to open the notification panel, tap and hold the Mobile data icon to enter the settings screen and turn on the VoLTE HD calls switch.

If the switch is not displayed, your device does not support the VoLTE service provided by your carrier. Also, some device models have the VoLTE service enabled by default, so the corresponding switch is missing.