Customizing system prompts is not supported and on an upgrade any customized system prompts will be overwritten.
In general it is not a good idea to customize system prompts as more than one conversation may share the same prompt and in one conversation the customized prompt might sound right and in another conversation it may not.
That said, in your specific case I think the risk is fairly low.
Assuming your system is using the English (US) prompt set and G711 prompts, the prompt you are looking for is in
\CommServer\Localize\Prompts\ENU\G711\CsSystemTransferScript
Do not ever delete a prompt as this can cause the system to go to the fail safe conversation "This system is temporarily unavailable...". The prompt.ini file in this directory gives you a text description of each prompt in this directory.
I believe you want SystemXferENU001.wav
I would suggest making a copy of the original prompt as a backup.
I would also suggest that you use the same prompt but simply add a couple of seconds of silence to the beginning of it.
One way to accomplish this is to open the Unity SA and use the MediaMaster to create a greeting. It allows you to copy from file.
First copy in some silence. I believe you can find a wav file that is just one second of silence in
C:\CommServer\Localize\Prompts\ENU\G711\AvSubMsgPlay
Check the prompt.ini file on your system, but on my version of Unity it is prompt AvSubMsgPlayENU129.wav
Copy this file into the media master a couple times to get a couple seconds of silence, then copy in the system transfer prompt mentioned above. now that you have your recording the way that you like, use the MediaMaster to save the file to your desktop. Grab the file from your desktop and save it the \CommServer\Localize\Prompts\ENU\G711\CsSystemTransferScript folder as SystemXferENU001.wav.
Again this is not supported, but this is how you could accomplish replacing this prompt with a customized prompt.
