How is the WiFi "permanently" turned off? Do you mean that you never enable it, did something happen to the phone that broke WiFi, what process causes it to be off "permanently"?
If your WiFi is permanently off, there's no way your call could have been WiFi. Having visited Verizon and AT&T, I assume T-Mo would have the same, they know who you are when you enter the store. Verizon puts you in a cue just by walking into some Corp stores with no input form the user. Your SIM tells them who you are, and, at least with Big Red and AT&T, my device automatically connected to their network.
You have missed or not answered some of our questions though and we need these answers.
Does the phone have an active account?
How is the WiFi for this device permanently disabled?
Have you tried any of the suggestions offered? Post #7 where you were asked to restart your router. Post #8 where you're given a list of items to check and do.
When you say the people at the store have turned it off and back on, do you mean the physical device was restarted or your service was interrupted and restarted?
Second.