Simply write a txt msg as you would usually, and then, when you go to send it, you need to put the 10-digit phone number (ie: including the area code)
Like: 0291234567 would send it to the person who lives in NSW with the number 9123 4567 (so they live somewhere in Sydney)
And 0758269134 would send it to someone who lives in QLD (I think) with the number 5826 9134
Very simple and easy and useful once you know how... Like when you have to RSVP to something and you don't have a voice and there is no e-mail address to use to RSVP... Sending a telstra txt to voice msg is very good...