Cannot send texts...

Hi, I activated my phone a couple days ago and no one has been able to receive texts from me. Calls work fine. I'm using AT&T StraightTalk on an iPhone 6s. Have tried turning iMessage off like support told me to and nothing...this is absolutely ridiculous. I don't want to pay for something only to not have all the features? Does anyone know what to do? The person on the support line was no help at all and was just trying to get me off the line. /: