I have free TFW service! How ?

I ported my number from Vz. I bought a Walmart family mobile sim and monthly service plan. Had a really tough time porting it. Took three days. Phone works fine. I have “TFW” showing as my carrier. Now here’s the rub. I have not paid for months and service still works. I can’t find my account at WFM or TFW. Not worst problem in the world to have. I’d like to upgrade to a new phone. I mean why not….since I’ve been saving a lot paying no one. Wonder if I can upgrade and keep the free service.