My iPhone 5 won't let me text certain people?

Alright, so on thursday night all of a sudden my text message wouldn't go through to this certain person. I tried resending it, and each time it would show the text message bar (right under the contact's name) saying "sending… " but once the sending progress bar reaches roughly 3/4th's finished, it'll just stop and the text message will put up an exclamation point (meaning that the message was not sent). I looked into it, and at first thought she blocked me, despite her saying she can't text me either. I assumed she'd blocked me and just left it at that and continued on. When I was texting another contact, the same things happened. I asked her and she too said that it wouldn't let her text me back. While researching this problem, I found out that maybe the IOS 6 software update that came out a few days ago was causing this problem as others have been experiencing it as well. Currently, it won't let me text those two people, but every one else works fine. I have resorted to Facebook messaging them, but I'd prefer texting since it's faster! Any advice or information is welcome. Thanks!

PS: I already edited their contact information, putting a "1" in front of the area code for long distance and that did not work.

Aha that happened to me on my iPhone… I called my phone company and they sorted it for me

Oh, i have that problem too! I don't know how to fix it either, so i can't help. Sorry!

Well join the club my friend, I have just upgraded to the Iphone 5 and changed networks, they sent out the wrong sim, it did fit, and got me up and running, but with lots of restrictions, when I contacted the new company ( Tmobile), they said I had the wrong sim and would send a new one, well now they have completely barred the one I have, and Iam hoping my new one will arrive tomorrow
so much for progress