T-Mobile has confirmed that beginning in January 2021, it will require all devices on its network to be VoLTE capable. T-Mobile issued a statement that it’s trying to reduce the impact of the change to new customers and won’t be activating new devices on its network after August 4 that lack VoLTE compatibility. It still puzzling, however,

Source: Hot Hardware – T-Mobile Confirms January 2021 Banhammer For Phones Lacking VoLTE Support