My Iphone5 won't let me send normal text messages? (tmobile)?

I just got the Iphone 5 in tmobile. And I can send imessages, but I can't send text messages. I can receive them, but it won't let me reply. It says "not delivered". Help. I already looked at settings and it says that the mms or what not is on, so I'm not really sure of the problem.

What is the best case for iPhone?

I don't want something bulky but I want something that will prOtect my phone a ton as in bulky I mean the survivor case I don't want anything like the survivor case

Unable to connect to iTunes on certain devices?

When I try to connect to my iTunes account on my iPod or my PC, it doesn't work. On the PC, it gives me error message 50 and says a network connection can't be established. On the iPod it just says "can't connect to iTunes store". I tried to connect my other iPhone and my iPad to the same account, and they worked fine. I know my internet is working. What's the problem and how do I fix it?

My iphone 4s can't connect to App Store?

When I try to download an app I enter my apple id password and then it says can't connect to App Store. It's starting to get really annoying. Everything else works. Like the Internet. I already tried turning it on and off and tiring off the Wi-Fi and turning on 3G and all that.