https://www.cnet.com/news/oneplus-6t-trade-in-deal-300-off-at-t-mobile/

That credit is applied over the course of a 24-month Equipment Installment Plan, meaning you'll pay just $11.67 per month for the 6T (not including service, of course).

Typically such trade-in deals give you maximum credit only for high-end/current-gen phones; the value drops with older models. Here, T-Mobile is giving you $300 across the board, for any of these phones: