Confirmed from T-Mobile chat support:
JOD customers will have two options, one of them chosen at checkout time based on account eligibility (factors include length of time as a customer and T-Mobile credit rating). Customers eligible for $0 down payment will pay $41/mo. Customers not eligible for a $0 down payment will pay $279.99 down, and their monthly installment will be $26 over 18 months.
tmobile has its own credit ranking systems for customers?