Jump only works if there is only an active EIP on the phone you're trading in.
Hey there JUMP is for when you are trading in a phone that you still owe money on and have paid at least %50 of the total cost. If you own your phone fully, you might be better off taking advantage of our 2018 Apple Trade In offer. If you trade in your 6s, we will give you $200 if it is in good condition. The credit would be in two parts, the first part would be a bill credit for the Fair Market Trade-in Value of your phone and the second part would be a rebate you would need to submit for.
As far as a down payment goes, this is going to vary based on your account as well as which GB option you choose. You would need to log into your my.t-mobile.com account and proceed through an order to see what that price is.