If you are using an Apple iPhone or iPad and activate iMessage, your Apple device automatically sends an international SMS in order to activate the iMessage service with Apple.
You will be charged $0.50c for this activation SMS each time you switch iMessage off and on.*
Please keep in mind that while using iMessage to send messages is free once activated, this uses data and therefore each time you send an iMessage, this will use your mobile data.
*UPDATE – As of 27/10/2020, there is no longer a charge passed through by Telstra Wholesale for iMessage activation.