Is the person in a good 3G network. I'm haveig the same problem. They say imessageing won't work when the phones off but it used to work every time. It's hard to know when its available but i have the block theough the iphone that if imessageing is busy it wont send buys it still does. I got charged for some when it was blue that somehow after i pressed send it turns green because my husband has iPhone 4 I have 4s.
I understand that different color messages may create some confusion. The reason your conversation messages may have alternate colors (blue and green) is because iMessage (blue) uses your data connection and a regular text message (green) uses the cellular phone connection on your phone and your text messaging package if you have one. If you don't have a text message you will be billed for each green message at a pay as you go rate ($.20 each) as indicated by gina7239 (thanks!).
iMessage will prefer to send it as a blue message using data connection, but if your data connection is not optimal, then iMessage will send your message as a green text via our cellular network. Keep in mind that if the person receiving your message is on an iPad or iPod Touch they do not have a cellular connection and will not receive your green messages.
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