Sprint says usage on non-Sprint carriers permanently bans phone from use on Sprint.

Sprint says all prepaid phone providers like Tello permanently change the status of something in the phone itself from postpaid (normal with Sprint) to prepaid. Tello says that data is in a Sprint database, not in the phone. Tello is apparently unable to change the flag.
Sprint tech support says "This renders any Sprint-compatible phone PERMANENTLY unusable on the Sprint network, since Sprint says Sprint cannot reverse this flag".
Personal opinion, I think Sprint techs are clueless. I've had them tell me there are no financial obligations on the phone, there are no flags or other issues with the phone, but when the purchaser of my phone tries to purchase Sprint service, they say "Gee, there's a flag which PERMANENTLY prevents this phone from being used with Sprint."
Note the phone was originally paid in full cash, then enabled on Sprint for a few months, then on Tello for a year. Clearly the phone is compatible with the Sprint network.

From the nthcircle forum:
If it was a Sprint Postpaid phone, it can be fixed to use on Sprint Postpaid. (Different story entirely if it started life as a Sprint Prepaid phone, though.)
Sprint tracks phones as PLBL (Private Label, or wholesale/MVNO) or SPCS (Sprint Postpaid)
Your phone should have been switched back to SPCS when released by Tello, but is still indicated as PLBL.

It's really just a matter of ticking a box in the database. but a lot of Sprint agents don't have a clue about what it is..
If it was last used on Tello, you could ask them for assistance-- their agents should understand this easily..

Tello says they cannot effect this change, I have to ask Sprint to do it.

But Sprint techs seem clueless and powerless to help the customer, even after different techs have told me there is nothing that prevents this particular phone from being used on the Sprint network.

Any suggestions?