'Messaging how to get the Delivery Status Callback to be called?

I'm using Messages to send a SMS, this is working fine. I have configured the "Delivery Status Callback" in Messaging, Services, "MY SERVICE NAME", Integration but the callback is not being fired. I have manually tested the Call back url from a browser and that works ok. What am I missing?



Sources

This article follows the attribution requirements of Stack Overflow and is licensed under CC BY-SA 3.0.

Source: Stack Overflow

Solution Source