Oh, I see! This is a real problem. Maybe the app could provide a neat user interface for framing a request, then send it as a text message to the call center, which then sends the answer back to the app.
I know this sounds contrived, but it seems to me the only way to not cannibalize the call centre is to not duplicate its functionality. The only thing left after taking the actual functionality away is the presentation, i.e. the user interface. So you can choose to use the call centre with the traditional interface (speaking on the phone) or the new app interface (that helps frame the question and later delivers the result).
I know this sounds contrived, but it seems to me the only way to not cannibalize the call centre is to not duplicate its functionality. The only thing left after taking the actual functionality away is the presentation, i.e. the user interface. So you can choose to use the call centre with the traditional interface (speaking on the phone) or the new app interface (that helps frame the question and later delivers the result).