Hi my name is ***** ***** to Just Answer. I am reviewing your question and will reply ASAP.
Have you got iMessage activated, or are you just trying to send a normal SMS message?
Tap Settings > Messages
What does it say for iMessage?
Try with and without iMessage enabled, and also make sure to put in the leading zero's in the phone number you are trying to SMS.
Let me know if you have any questions.