intercambio

When I became a new tmobile customer, I was told I could trade my phone in  after 1 year. This was one of the perks of tmobile. Unfortunately, I was never told specifically how it works. I have been trying to trade my phone in for the past 6 months (been with tmobile for 2 yrs now). Im eligible for a trade in up to $1000 but tmoblie wont allow me to use my trade in towards the new phone. I was told the $1000 for go towards some credit and I have to pay for phone up-front. This system was NEVER explained from the beginning. I already filed a complaint via BBB. How can I get my trade in to go towards my new phone? Who do I need to talk to in order to fix this situation?