Rut200 post/get sms

I’m testing the POST/GET option in the SMS Gateway.

What is the syntax for deleting one specific (or possibly all) SMS ?

Tried this:
http://192.168.178.12/cgi-bin/sms_delete?username=TestUser1&password=TestPass!1234&index=1

Result is: wrong_number

Thx for clarification.
Gerhard