A Call back URL is created so that the user’s data is delivered directly to that URL.  For example, if a user wants to post data and send us a message then it will directly hit that URL. All information is linked to that URL so that whatever information we want to access about a user is shown in that URL. 

With ABTXT.COM you can receive SMS Delivery Notifications and Incoming Text Messages Via Callback URLs. You can set up callback URLs to get the delivery status.