Plug the USB cable into your computer or mobile device. With the receiver handset in the cradle it will not show up as a USB HID device.
Lift the receiver. The board will restart, this time with USB HID enabled, thanks to the boot.py file code. On a mobile device such as the iPad, this will cause the virtual keyboard to disappear, indicating that the device is ready to receive "keyboard" input.
Dial numbers on the rotary dial, and they will be sent to the device over USB!
Hang up the phone when you're done to resume virtual key entry.