T-Mobile will stop activating phones on its network that don't support Voice over LTE (VoLTE) starting on August 4th of this year, and existing phones that aren't compatible will completely stop working on T-Mobile sometime in January 2021. The move doesn't give customers much time to replace their devices, especially if they're using a flagship phone and don't want to use something slower.
Thankfully, it's very easy to find out if the phone you have supports T-Mobile's implementation of VoLTE. We'll show you how.
Check the Settings app
If your phone is already activated on T-Mobile, you can check in your phone's settings if VoLTE is being used. Open the Settings app on your phone and go to the 'About phone' menu, then tap on 'SIM Status'. If you can't find it, try pressing the search button in the app and typing SIM.
Once you find the SIM Status option, tap it, and a popup should appear. If the voice network type is listed as LTE, you're good to go.
If your Settings app doesn't have that menu for whatever reason, try dialing *#*#4636#*#* in your Phone app. This will take you to a hidden diagnostics menu. Select 'Phone information,' then check if LTE is listed under Voice Network Type.
Check your IMEI number
If you're not already a T-Mobile customer, you can still check if your phone supports T-Mobile's VoLTE with the company's compatibility page. All you need is your device's IMEI number, which you can find by searching for "IMEI" in your phone's Settings app.
Once you have the number, put it into the text box on T-Mobile's compatibility page, and click the big Check phone button. The site will then tell you, among other things, if your phone is specifically compatible with T-Mobile's VoLTE.
Unfortunately, the two above methods won't help you if you don't already have the phone in your possession. There's not an easy way to check before purchasing an Android phone if it will support a specific type of VoLTE, especially when nearly every popular Android phone has different carrier-branded variants that don't share the same network features.
Generally speaking, most carrier-unlocked Android phones sold in the United States should support T-Mobile's VoLTE. The carrier-unlocked component is crucial — phones sold for one specific carrier might not support the features of another carrier, even after it is unlocked for use with any network.
T-Mobile has yet to publish a comprehensive list of supported devices, so for now, the only way to be 100% sure if a phone will survive the shift to VoLTE is to test it yourself.