Forced to "send as SMS" then don't receive replies
I just upgraded to a 5c, with carrier AT&T. Recently I've been trying to send texts (iMessages) and after a few failed tries it asks if I want to "send as SMS", so I do. The four times so far that I've had to do this, I haven't received any replies to my texts. I asked one of my friends, who confirmed that she got it and sent me a message back, but i never received it. I'm not sure what to do about this, because it looks like my texts ARE being received as SMSs, but then I do not see the replies. Any insight?
iPhone 5c, iOS 7