Why isn't my iMessage working

Okay so I went away for a few weeks and now that I'm home I want to use iMessage on my phone again (iPhone 3) but it was off so i just thought that it turned off automatically when I went overseas but when I turned it on again it wouldn't work; it just sat there with "waiting for activation… " on the screen and not actually working. So I'm just wondering WHY this is and how I can fix it?
Cheers.

How much will I get if I sell my black iPhone 4?

My 24 month contract is coming to an end in the next month. I want to sell my iPhone 4. I've never dropped it and it has no scratches on the screen, as I have always had a screen protector on it. There are only light scratches on the back, due to normal wear and tear. There's nothing wrong with anything about it, never had a problem. How much will I get for it?

(I'm in Australia.)

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I have just recently jailbroken my iPhone 3GS and unlocked it for use with T-Mobile. The Wi-Fi works, data works, and I can call with my phone fine; however, whenever I try to send a text message to ANY ONE, it fails to deliver (I get the red exclamation point). No matter how many times I click "try again", the message fails to send. Is there a way to fix this! Also, the camera option next to the text box is "greyed out" so to say; that is, I can't click on it at all so I would have to go to my camera roll and attach a picture instead of just taking one then and there. How can I fix that as well!

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When I make a call on my iPhone 4 my screen is still bright ', its not automatically turn off by sensor. Now how to turn on sensor on my iPhone 4