Most likely you got it subsidized by your contracted service plan. T-Mobile actually makes that much more clear up front than most providers do: if you bring your own phone, you can sign up for the "Even More Plus" plans which cost less per month and have no long-term contract, but if you want a subsidized phone you need the "Even More" plans which cost more per month and require a 2-year contract.

So, that "free" phone costs you $10-$20 extra per month, for 2 years.