Phone help??!

My DF was at work yesterday and kept texting me asking why I wasn't responding and asking if I was ok. I was texting back every time. Well, when he got home we realized that he wasn't receiving any texts at all. He can send text messages, get online, make and receive phone calls. But for some reason he isn't getting any texts?? I tried calling AT&T customer service but they weren't any help.. :( has anyone had this problem and how did you fix it??