Australian residents now have a real sms gateway to take advantage of! No amount of advertising banners could beat this deal, 1000 FREE SMS's just for signing up!. The API already features a huge number pool and allows for reply's. The Telstra developers are extending the API's they currently offer and have a few awesome features on the way.
Head over to the Telstra developers site and create your app, grab your API key and get texting.

Usage:

- Access tokens are valid for 1 hour so store a timestamp and have a rule to grab a new token if the hour is nearly up or past when a user next makes an API call.

- When you send an SMS, store your message ID.

- Use the message ID to check the delivery status of the SMS.

- Use the same message ID to check for reply's to the corresponding SMS.