Can't send message via SMS Center


I am quite new in ERPNext. I wish to use ERPNext to send SMS to customers. For SMS Settings, I already fill in required parameter and URL as figure below:

But when I click “Send SMS” button in SMS Center with recipient contact number in receiver list and message, the recipient did not received any message although I am prompted with SMS sent to following numbers: 601xxxxxxxx and able to view message sent in SMS Log.

Did I missed any step? Can anyone help me?

Thank you