Good suggestion, ajzwilli ! Yes, Ting can definitely do this in most circumstances, since they apparently have direct access to the Sprint database. I think being either a current, or former, customer may be a requirement, and that's more than fair in my view. The $6 minimum monthly fee might be a reasonable expense. I have seen reports of folks intentionally running phones through Ting to fix errors.
I'd try Tello first, since Tello was the recent provider. I don't think they have quite the capabilities of Ting, but they should feel an obligation to assist if possible.
I ran the phone through Ting. Opened then cancelled a new phone #. Seemed very helpful and capable.
Ting says Sprint should now accept the phone.
Edit: From Ting chat support "at this point Sprint should be able to use the device."
I'm about to find out if Sprint can help.
Alexander from Ting saved the day for me!
After being told by Tello that they were unable to help, and being told multiple times by Sprint that phones could never be used again on postpaid Sprint networks once they were used on MVNOs like Tello, I followed the advice of azwilli I created a Ting account, then asked Alexander via Chat to help. Alexander gave me a new phone number then cancelled service, and switched the flag so Sprint could use it with their postpaid plans.
The new owner is now a happy user on a regular Sprint account.
Boo to Tello for their lack of interest and/or capability to help.
Hurray to Ting for their help.
Glad it worked out for you. Ting's customer service has been unmatched in my experience. I was with them for years before switching to RingPlus and then to CellNuvo. If there were more competitive on price for my usage I would have stayed, but since I don't need customer service that frequently, I left to reduce my costs. Not sure what I'll do after my free year with Sprint ends, but the competitive landscape will likely change by the end of this year.
Glad to hear you finally got a solution, jackhobbs !