In many ways, CDMA was better for voice phones, especially in regard to power consumption. There are two reasons why the battery of a Sunbeam phone running on VoLTE will not last as long as a CDMA phone running an older operating system.
The first reason is that the BasicOS operating system is based on AOSP, which is a much larger and more complex operating system. This is like a V12 engine compared to a four-cylinder engine in older phones. Even when idling, it still consumes more power.
The other reason is that the VoLTE signal is a packet-switched connection and requires far more processing than the older circuit-switched CDMA signal. Both of these things mean that the good old days of batteries lasting all week are gone forever unless you have a massive battery.
Having Wi-Fi or Bluetooth turned on will also drain the battery more quickly.
To turn off Wi-Fi: System>Settings>Connectivity>WLAN>turn Off
To turn off Bluetooth: System>Settings>Connectivity>turn Off