TracFone might be on the verge of adding T-Mobile to its Bring Your Own Phone (BYOP) program. Customers can currently use Verizon, AT&T and unlocked GSM phones on TracFone. Verizon phones on TracFone operate on the Verizon network and AT&T and unlocked GSM phones use AT&T's network. But currently there's no way to use a locked T-Mobile phone TracFone.
That might change soon. If you go to TracFone's BYOP sales page at tracfonewireless.com/byop/ and click on the button next to the words "If you have an AT&T or unlocked GSM smartphone, you're compatible! Check your compatibility here." You will land on the page pictured above. That middle T-Mobile box is new, at least it wasn't there the last time I checked a week or two ago.
If you click the T-Mobile Compatible Phone button you will be prompted for a zipcode and taken to a page with the options to buy or activate a SIM. If you choose to buy a SIM you'll be asked to choose which size SIM you need. Choosing a SIM currently lands you on an error page. It looks like TracFone is in the process of adding T-Mobile but they aren't finished yet. This is pretty typical of TracFone's haphazard approach to web development and the technical side of their business in general. New products and services often appear as non-working web pages before eventually going live.
If you are wondering why TracFone would want to offer T-Mobile BYOP, I think there are two reasons. For one, TracFone is going after the market of potential customers with locked T-Mobile phones, some of which like the LG Leon and LG G Stylo, are difficult or expensive to unlock. In addition, it's widely believed that it's more profitable for TracFone to offer service on T-Mobile than on AT&T or Verizon.
The advantages of T-Mobile BYOP to users are less clear. T-Mobile has considerably less coverage than AT&T or Verizon and TracFone has not offered roaming on their BYOP services on any operator. The main pluses of T-Mobile BYOP to users are potentially faster LTE speeds than AT&T in some markets and the ability to activate locked T-Mobile phones.