Your message has been sent is a default system prompt, and any Cisco Unity system prompt can be replaced by a customized .wav file.
To turn off the Your message has been sent interview handler, perform these steps:
Go to the D:\CommServer\Localize\Prompts\ENU\G729a\AvPHInterview folder, and rename the file AvPHInterviewENU002.wav to AvPHInterviewENU002.old, and paste a blank file with the same name.
The blank file can be found in the D:\CommServer\Localize\Prompts\ENU\G729a\AvAddrSearch folder. The file name is AvAddrSearchENU002.wav. This file must be copied to the AvPHInterview folder. Rename it to AvPHInterviewENU002.wav. You are now able to remove the Your message has been sent system prompt.
Note: Any Cisco Unity system prompt can be replaced by a customized .wav file, but that change is not supported by Technical Support if any problem arises in future.