I just pop the new TPG simcard into my Redmi Note 4, confirm can volte call without the TPG 4G voice app. Put the same TPG simcard into the Vivo 1820, cannot volte call out unless use TPG 4G voice app. I try to find under the settings -> mobile network but cannot find option to activate VOLTE. I try to find solutions in youtube. They show volte option but my Vivo 1820 doesnt have this option...tpg uses volte
redmi note 4 supports volte.
Vivo 1820 should support too, go to settings -> mobile network to turn on.