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.
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.
The option to "send as sms when imessage is unavailable" refers to the sender side, meaning that if the sender is not connected to iMessage (i.e. offline), the message will be sent to the receiver as a SMS.
Example: Sender is not connected to the internet / have bad internet connection and cannot connect to iMessage server, then their message sending to a registered iMessage receiver will be sent as SMS instead.
iMessage will not be used at all if the receiver is not an iMessage user.
iMessage will be sent if the sender has connection to iMessage server but the receiver does not have connection to iMessage. The receiver will receive the message the next time they connect to the iMessage server (when their connection is back).