Google just announced a feature that allows you to send a txt message to the person you ignore the call of with precanned messages. Is there an app for that on the iPhone? Probably my fav feature I saw for icecream. 
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And I just answered my own question but need to jailbreak.
http://iphonemonsta.com/imbusysms-swipe-ignore-call-autosend-sms-busy-message-cydia-tweak
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And I just answered my own question but need to jailbreak.
http://iphonemonsta.com/imbusysms-swipe-ignore-call-autosend-sms-busy-message-cydia-tweak