Turn On Data Roaming SAMSUNG Galaxy S21 Ultra, how to - HardReset.info
SamsungS21Series | Facebook
Do NOT update to One UI 4 on Samsung S21, if your carrier relies on VoLTE and wifi calling exclusively for phone calls, SMS(May affect other models too) : r/samsung
Solved: VOLTE settings - Samsung Members
How to enable VoLTE on a Samsung Galaxy phone | Android Central