I've noticed this has become more of a problem in switching between iOS and android mainly because of ios7. By default if imessage is active, iOS7 unchecks 'send as SMS' if imessage is unavailable. This causes problems in sending messages between people who previously had iphones and made the switch to android. In order to fix this have your family/friends etc make sure they go to settings>messages and then check the box 'send as SMS'. This, in combination with deleting old message threads should solve your problem. Now the phone will recognize that imessage isn't available to that user who made the switch and it will send as a regular SMS instead. I also made the switch from an ip5s and had this issue and this seemed to solve it!
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