Well, here you are what I think of:
1- You need a master chip. Something like the Max3421E. It's a USB host controller which will be the master of the system.
2- You need a microcontroller that will communicate with both the mobile phone and the camera.
3- A mobile phone application to that will communicate with the camera through microcontroller.
Yes, you need a master or host-controller. FTDI made a chip for this purpose specifically to allow embedded devices to control USB slaves. I have not had an opportunity to use it yet, but it looks neat. Below is a URL: