Moto X At&T Straight Talk Help

Hello everyone, I currently bought an unlocked Moto X AT&T phone and I've bought a Straight Talk sim card. I am able to text just fine but my problem is that I can make any calls. I've called Straight Talk several times already and yet till I'm not able to call or receive them. Please can anyone help me fix this problem.

In Settings>wireless and networks, you should have an option More... at the bottom. Click that, and that should get you to a list of options including Access Point Names. See if changing one of those might help.