Unknowingly I purchased a Att Galaxy s6 active, I want to use Family Plan Walmart

I really need to find out if I can salvage this phone I purchased from an aquaintance. A few days after purchasing this phone I then purchased a Family Plan Sim Card from Walmart. I then went to install it and the comment thay my phone flashed back at me was locked network on this phone and that the sim was the incorrect one. So I called ATT and was told to call back and see about a unlock code. I wanna know asap if possible if Im wasting my time to use this phone or if I can switch sims how please how!!?

from xda-developers http://ift.tt/28IHQh9
via IFTTT