The BB trade in program is part of the device recovery program which is only available through
post paid plans.
http://www.t-mobile.com/DeviceRecovery/cell-phone-trade-in.aspx
Limited time offer; subject to change. Device Recovery: Qualifying postpaid plan required. Trade-in qualifying device for credit. Credit applied to existing Equipment Installment Plan(s) (EIP) balance(s) or to customer bill in the order determined by T-Mobile and customer eligibility; may take several billing cycles to receive credit. Device recovery transactions are final; after you trade in your device, you cannot get it back. CWork may make adjustments to the Initial Quote based upon evaluation of quality and condition of the device on receipt. You can trade-in one device per active line at the time you upgrade or activate a new line of service. See Device Recovery Program Terms and Conditions for details. Device and screen images simulated. ETF Reimbursement Offer: Limited time offer; subject to change. Eligible device trade-in, new device purchase, qualifying credit, port-in from eligible carriers (incl. AT&T, Verizon, and Sprint), and qualifying postpaid service required. Payments consist of: (1) credit of device trade-in value, and (2) prepaid MasterCard card in amount of Early Termination Fee. Tax on ETF not included. You must submit final bill showing ETF within 2 calendar months of port-in and be active and in good standing with T-Mobile when payment is processed; allow up to 8 weeks. Additional validation may be required. Up to 5 lines; all lines must be activated in same T-Mobile market with same billing address. One reimbursement per subscriber. Check your contract with your carrier for your rights and obligations.