purchased locked phone.

So my girlfriend just purchased an S6 (sprint) and when she took it to the store to have it activated they couldn't do it because it was "locked"...some kind of carrier lock I'm assuming.. Like they owed sprint money or something? Obviously if it was a normal security lock than we could just do a factory reset but apparently that won't work.
So.. Anyone know anything about this and a way around it? I'm a Verizon user and have never heard of this kind of problem. Thanks for any help in advance.