It could also mean your iMessage needs to be completely activated.
Try going to:
Settings>Messages>iMessage(Checked On)>Send as SMS(Checked On)
Once you've verified those settings then go to:
Settings>General>Reset>Reset Network Settings
The device will power cycle and you may be required to re-enter any wireless network passwords that you had stored, but otherwise, this should get your iMessage and SMS issue resolved. Best of luck 