In response to the original post from ywid062 who asked:
"what are the costs associated with reading sms replies? 1c? A bit less clear on this on the PAYG pricing page"
We have addressed that in our FAQs:
You can send and receive messages via the API by using the notifyURL feature. See the documentation for more info: https://dev.telstra.com/content/messaging-api#operation/retrieveSmsReplies
You will not be charged for any inbound messages to your virtual mobile number retrieved via GET /messages/sms. However, receipts via GET /messages/sms/{messageId}/status are charged (first read only).
Please let me know if you'd like further clarification!