Charged international SMS for answering a national landline phone call with a message
A month ago I received a phone call from a contact one day at a meeting. I was not able to take the call; as my iPhone showed up an option to text the caller, I selected that option to send a message "Sorry, I can't talk right now." to the fixed line number, as I didn't realised my contact was calling from a landline number instead of a mobile phone.
Today when I received my bill, I found a charge for international SMS, which is unusual, so I checked my SMS history and found the offending message was that "Sorry, I can't talk right now", and it was sent to 0011 2 9xxx xxxx. The landline number is in a NSW number 02 9xxx xxxx.
Is this an incorrect charge? What can I do to avoid sending SMS to landline number in the future, other than paying more attention to the number of the incoming call?