My unlocked LG X-Charge purchased from Amazon (Unlocked LG X charge from Amazon Prime (US601) | LG USA) years ago was used on R+ (I think), Ting, FreedomPop, and at one point, Tello.
Today, Tello's IMEI checker site says: "Oops! Your phone is not compatible with our GSM network. Please note that you need a compatible device in order to use our service."
As a device originally released in 2017, it might not be T-Mobile VoLTE compatible. Can you check the IMEI at the following site and report the results?
T-Mobile IMEI Check
Click Question Mark After Entering IEMI For Detailed Info
Thank you for the mint mobile checker. The checker reports incompatible.
So looks like VoLTE is the standard. I'll see if I can find a data-only plan & SIM to continue using this phone. Or, it's time to replace it.
To avoid buying another phone for Tello, I found that both the Tello and Mint checkers indicate the ZTE Blade A3 Prime (from Visible) is compatible.
What's a good way to determine if it's unlocked?
The checker is wrong. For the most part, the ZTE A3 will only work as a data-only device on T-Mobile. It cannot do VoLTE on T-Mobile. Also, Visible locked out 2G and 3G GSM voice/text capability on the A3. For all intents and purpose, the A3 only has full phone capability on Visible and Verizon.