Responses to your outgoing messages can be picked up in a few different ways. Your DLR Callback URL in your account settings, is the default URL we post incoming DLR's to. You can also set the DLR Callback URL separately for individual messages, using a parameter on the send-sms call. You can also poll the following endpoints for responses.

Language
Credentials
Basic
base64
:
Click Try It! to start a request and see the response here!