Voice Over Long Term Evolution is a means in carrying voice calls over the 4G network rather than the old 2G or 3G network. This standard is required in order to shut down the obsolete networks, and make it so a network provider will only have to deal with two types of networks. VOLTE also has benefits to the consumer. A higher quality voice call is made making the calls more clear, and better sounding. While not essential for a phone to work, it is still a benefit.
All current cellular phones must support VOLTE. If someone pulled out their old 3G phone in hopes it would work, it will not. This is because the carrier likely completely shut down the 3G network once 3G phones has become a certain age, and any new upgrades would have been VOLTE. Even basic phones uses VOLTE to make and receive calls even if they don’t use the conventional data services with the phone.