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?
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.