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There have been several changes in the new iOS 7 and this is one of them just read the article on cnet

Code:
http://crave.cnet.co.uk/mobiles/imessage-not-working-for-iphone-and-ipad-after-ios-7-50012388/
 
If you travel internationally, texts range from 50c to $3 per text. There are messages that I would send through iMessage (using cellular data) that I wouldn't want to pay 50c-$3 back and forth. Mainly conversations that could wait for wifi...
 
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