LG X-Charge listed as incompatible

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."

Thoughts?

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
https://www.mintmobile.com/byop/
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.

Thanks!

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.