Sprint has taken their sweet time about it but it looks like they have finally started to fix the mess that they created. Sprint MVNO Ting posted an update on their blog reporting;
Progress! More than 30 million devices that would previously have failed Sprint’s financial eligibility date (FED) check will now pass.
Now, devices that were freed from a contract due to a hardware upgrade (that is to say, you got a new a new phone on the same contract) will clear the FED check. Likewise, with the exception of the iPhone 5S and iPhone 5C, any device that was activated 12 or more months ago, regardless of whether it’s currently active, will clear. This applies to devices associated with postpaid accounts only.
If you tried to bring your device to Ting previously but failed due to FED, please try again.
With many Sprint phones still failing the FEC, prospective Sprint MVNO customers should always check that the ESN or MEID of any phone they intend to buy passes the Ting ESN checker, RingPlus checker or FreedomPop checker. Of the three checkers, Ting and RingPlus' are the easiest to use because they dont require you to enter your email address like FreedomPop's checker does. A phone that passes any of the three checkers should work on any Sprint MVNO.
If you have a phone that you know has no financial obligation to Sprint and it fails the Ting, RingPlus or FreedomPop checker, I recommend filling an FCC complaint saying that Sprint is blocking you from using your fully paid for phone from being used on another carrier. That's the only way I know of that an individual user can bring any pressure on Sprint to fix their still broken BYOSP activation process.
Related: Updated: Sprint MVNO BYOSP is Badly Broken