Meaning of "send as sms when imessage is unavailable"? I want the option to "send as sms when imessage is unavailable" to turn off. But if it's on, does that mean it sends as SMS when imessage is unavailable on MY end (the sending end), the receiving end, or both? Example: while I had this option on, if I texted a friend whose phone was on airplane mode or off, it would go through as sms and not imessage. With this option off, if I imessage someone and their phone is on airplane mode/off, would it send as SMS because imessage is available on my end? Or keep it as an undelivered imessage because imessage is unavailable on their end??
This option means when a user with iMessage doesn't have an Internet connection or iMessage is disabled.

For example:
One day while chatting with a friend via iMessage your friend goes into a tunel and disconnects from the Internet, your iPhone detects that your friend is off-line. If the option "send as SMS when iMessage is unavailable" is enabled, 
the messsage will be send as an SMS. If the option "send as SMS when iMessage is unavailable" is disabled, the message will only be sent as an iMessage when your friend comes back online.