Response Codes as below. These vary depending on which country you are sending reminders to and which underlying provider Appointment Reminder uses to send the reminders.
Check the number is correct and in International Format and then resend. The error string will also provide some more useful information such as whether the recipient is blocked. Be careful about infinite loops, only retry a couple of times.
Message Passed to Network
Wait. Give it a few minutes and then poll for more Status Messages.
An error occurred. It may be recoverable so try sending the message again in a few minutes. Be careful about infinite loops in your code.
NRCV or NRC
Check the number is correct and in International Format and then resend. Be careful about infinite loops though.
Excellent. This is a final status for this message and the handset has received it.
Message was not delivered within allowed time
This is a final status for this message and the handset has not received it. Retry if you want but for a lot of carriers this status may take days to return so the message may no longer be relevant.