The big companies like Amazon, etc already have this or could easily add it. I think this would be more useful for the thousands of other smaller companies out there. Perhaps you can wigitize this product and offer companies to place it on their website. I can see the longtail working out better.

On the other end, as far as I know for something like Amazon, etc you need to call in order to be put in the line for them to call you back. The cool thing about your service is I don't need to call :)