I've been getting text messages from this phone number for several months. My phone, however, says that "message cannot be displayed". When I call the number I get an intercept saying it is not a working number.

i believe that this is the AT&T message center number that routes text messages. It is not the actual number that is sending you a text message. All your texts should have this number, but you should also see the actual number of the user who sent the message.