Hi,
You can try New-CsRgsCallAction to set Prompt.
Prompt to be played before the call action takes place. (For example, "Please hold while your call is transferred.") Prompts must be created by using the New-CsRgsPrompt cmdlet.
http://technet.microsoft.com/en-us/library/gg398136.aspx
The Example 1 is used for OverflowAction, you can change it to TimeoutAction.
Regards,
Kent