If I HAD to pick
The Verizon model of financing it over the two year contract, tho I realize they only break it up over 12 months. I would modify it slightly to be like a car lease. While your paying the monthly data fee on the device and the financed device cost. If say month 8 in the 12 month device finance you wanted to end services, there should be the option to surrender the device to avoid the ETF, also require insurance be carried on it and thus you've hedged your loss on the customer destroying it to avoid etf. I propose a model like this: 19.99 for data (amount to be determined) 9.99 Insurance, 24.99 Device Installment. Comes out to around 55/month, nets Verizon more data subs, moves more devices, and slightly raises ARPU.
Odds are those that can qualify for zero deposit to start service would love this, and I'd even would permit $100/$200 range deposit customers to finance. Because after a few months into the service, if they non-payed, VZ is covered, they could hand over the device and the cust avoids ETF, and VZ gets its asset back, to then resell as a CLNR/Refurb/Prepaid device plus would have the few hundred on the deposit to eliminate the non-pay bill in most cases. Worse case is cust never hands in device, and gets hit with ETF, or files Insurance claim if device is damaged, restoring VZ whole, and still has the deposit to offset any debt.