VOLTE is a feature which is used in 4G - LTE. Basically VOLTE means when you make an voice call , voice signal and data is transmitted via LTE network.
If you just purchase 4G-LTE phone then voice call will be transferred via 3G or 2G network. Disadvantage of this is if the operator doesn't support 3G/2G network then you cannot make a call. So better purchase VOLTE phone