Invalid ESN? What does this mean?

I know what a bad/blacklisted ESN is. However when I checked the esn of a Sprint iPhone 5 it said that ESN is not valid and to contact customer service. Usually if I check an imei or esn of a phone and it doesn't work it will say "reported stolen or missing". However it just said it was not valid. Does this mean the friend I bought the phone from has to terminate the service with the phone? I'm not sure how Sprint does there phone activations, so can someone please help? It would be greatly appreciated.

It means that cell phone is tied to your friends bad account. It could be that there's a balance still due on the account or the account is in collections. For this reason, Sprint will never reactivate any phone that has a bad account. You should call your friend and tell him or her to contact Sprint to see what the problem is.