I have an iPad with iOS5 and I've set my apple ID with iMessage (Messages). I can easily send messages to other iOS devices set with Email addresses. But I can't send a message to people who use iPhone. In fact, when i type someone's phone number (who i know uses iMessage with that phone number) or add their contact, after a little thinking, a red exclamation mark appears next to the number and it says "[number] is not register with iMessage". But if that same number texts me with iMessage, I can reply their message. Can anyone please help me?! It's driving me crazy...!