Help! Bought iPhone off craigslist.

[_ceals]

I bought an iPhone 5c (Sprint) off of craigslist a few days ago to replace my current phone (Boost Mobile) but I am looking to keep Boost. I went to boost to get the phone switched and when I called to get the IMEI released, they said it was still registered under an account. I don't know the informatjon for this account and am not sure what to do past this. The guy I bought if off of is not returning my calls. There is no iCloud or Find my iPhone locks or any other physical locks on the phone and so far as I know the IMEI is not black listed as stolen or lost, but I can't be sure. How can I get past this? I'm willing to try anything, but need to keep my boost number. Thanks in advance!