How do I unblock a number on my iphone?

I just recently got an iphone. Somehow one of my friend's numbers got blocked. My friend was able to call/text me after I got the new phone, but starting about a week ago has been unable to call me. Everytime they call, it doesn't even ring and they hear "The AT&T subscriber you are trying to reach is unavailable" and then it hangs up. I can't get any of the text messages they send either. They are able to call other AT&T customers. I can only conclude that somehow the number got blocked (although I didn't think you could block numbers). Has anyone heard of this happening? Does anyone have any thoughts or suggestions (other than calling customer service)?! Thanks!

Didn't know you can block people on the iPhone. Have you called ATT yet and asked about it?

never heard of that one, you will have to call customer service

Yup I would call AT&T.

Call AT&T.

I had an issue where I couldn't receive a text. AT&T Tech Support straightened it out.