If you’re a Sprint customer and signed a contract between July 1999 and December 2008 you might be eligible for some cash. If you were charged an ETF you are eligible for $90. You have to provide proof.
If you didn’t cancel for the purposes of avoiding paying the ETF you could be eligible for $35. You need to jump through the legal mumbo-jumbo, but you can do the paperwork at this link and read all about it here.