Your current mobile phone probably supports Voice Over LTE (VoLTE), a system that lets calls go over the data network rather than the old-fashioned dedicated calling system. It is good, which enables much higher quality sound. According to a report from Android Police, T-Mobile will start using this method to call exclusively in January.
This means that from the beginning of next year, T-Mobile̵7;s network will simply not work for calls on older phones that do not use the VoLTE system. According to the leaked internal documentation, all new devices sold by T-Mobile will support VoLTE starting next month, and all devices authenticated online starting next year will need to support it. Incompatible phones cannot make calls, or may not be able to access the network at all.
T-Mobile is probably doing this in preparation for shutting down older 2G and 3G networks, freeing up wireless spectrum for more LTE and 5G services. This will not affect most customers, as the vast majority of mobile devices sold in recent years already support VoLTE. It can be a headache if you try to use an older device, or if you use a T-Mobile SIM in a low-cost phone purchased for travel. Similar moves are planned by other carriers.
When Android police asked T-Mobile to comment on the leaked internal documentation, T-Mobile declined, so it is possible that this plan is still tentative.
Source: Android Police via 9to5Google