Why won't my Nexus 5 (Lollipop) work on AT&T?

[AC Question]

I was just at the AT&T store today trying to activate my phone on their network and they were unable to get it to work. The phone would connect to the data network with no problem, but when they would test the calls they could not get it to work. They dialed a number and hit the call button but it would show it dialing and then instantly show "call ended" with no ring. They told me it was probably because the phone was purchased from Sprint and would need to be unlocked. I did call Sprint to get the unlock code, which they did give me. Any idea if this will fix the problem? Or is it a different issue? I've read on some forums that the Nexus 5 has problems working with AT&T's new VoLTE. Any ideas?