Your current cell phone probably supports Voice Over LTE (VoLTE), a system that allows calls to go over the data network rather than the old-fashioned dedicated calling system. This is a good thing, because it allows for much better sound quality. According to a report by Android Police, T-Mobile will start using this calling method exclusively in January.
This means that at the start of next year, T-Mobile’s network will simply not work for calls made to older phones that do not use VoLTE. According to the leaked internal documentation, all new devices sold by T-Mobile will support VoLTE starting next month, and all devices that authenticate to the network from next year will have to support it. Incompatible phones will not be able to make calls or may not be able to access the network at all.
T-Mobile is presumably doing this with a view to shutting down the old 2G and 3G networks, freeing up the 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. This can be a headache if you are trying to use an older device or if you use a T-Mobile SIM card in a low cost phone bought for travel. Similar movements are planned by other carriers.
When Android police asked T-Mobile to comment on the leaked internal documentation, T-Mobile refused, so it’s possible that plan is still tentative.