I get at least one call a day from NCO Financial Services so I don't answer my cell phone any more. I just let it go to "Missed Call."
When you file a complaint with the FTC, include the NANPA information.
In this case: NANPA: Assigned 0000-00-00 Wshngtnzn1, District Of Columbia (Sprint Spectrum L.p.)
It can't hurt. It might help the FTC build a case against them.