The same scenario works perfectly in the CPMInteractionPortalHeader section/ the PopUp of the incoming Call... except in the body of the portal.
This is not standard behavior so I would suggest that you attach with a debugger and make sure that you are able to really get to the correct Frame object and call the function. We do this for Chat interactions to get the dialogs to automatically populate the chat window. So, it is doable. You just need to make sure you find the correct iframe object.
The iframe was used in the CPMInteractionPortalHeader. So I had to use this syntax to call the function from a section of an interaction: "window.parent.frames.iFrameName.FunctionName". Its context was in the parent window.