Iphone 4s why can't i use it?

Iphone 4s why can't i use it?!

Added (1). the iphone 4s is locked to the sprint service. Its connected to the network, full bars, and shows 3g, but i can't use web, call, nor text. When making a call I get a busy tone. Also id like to change the number in the phone as it was not my phone. There's no way to go into settings and change it as it appears grey and can't be changed. I've also restarted the phone multiple times as the number seems to not disappear.

Its blacklisted. Also Sprint has sims embedded in so that number is forever.