DLRs (short for delivery reports) are status reports that hold information about the status of a message that you have sent through our API. Status reports are requests that are sent to your platform through a GET request. Status reports are only provided for SMS messages that have a reference and accounts that have configured their status report URL.
You can check the full documentation on handling status reports here.
Keep in mind that it is not possible to whitelist the IP range for our REST API since our IP addresses are dynamic. However, you can create a sign-in key to verify HTTP requests. You can read more about this in our authentication documentation.