Trade in an iPhone 6S or newer IN ANY CONDITION (according to terms) and lease a new XR for 18 months for $0/mo after $31.25 monthly credit
"Bring your old iPhone 6s or newer in any condition to Sprint and lease a new iPhone XR on us.
iPhone XR 64GB $0/mo. after $31.25/mo. credit, applied within 2 bills. With approved credit, 18-month lease & new line of service. If you cancel early, remaining balance due. If you cancel early, remaining balance due. Excludes tax"
After 18 months you can keep the phone by paying off balance of $187 (or so) or simply return it and owe nothing.