Hi @DrVikram, welcome to the community.
Check that you have VoLTE set up.
Find and tap Settings > More > Mobile networks > Preferred network type > LTE or 4G (preferred)/3G/GSM.
Drag the slider beside Enable VoLTE to the left.
If you do have it enabled, or you do not have the option, because VoLTE is supplied by your network operator, they need to customise the update for your phone to use it. I recommend you contact them in regards to this.
