Switching between systems on the iPhone is performed by double tapping a specific region of the screen.
The top of the screen contains a navigation bar on almost all screens. Double tapping the middle of this bar will allow you to switch between systems.
If only 1 system is configured nothing will happen. If only 2 systems are configured eKeypad will automatically toggle between the two systems. If 3 or more systems are configured eKeypad will present a popup allowing you to choose the system.