You would need to call it after it is attached. Many properties require the device to be attached before setting them. This can be done either in the AttachHandler or once you know it is attached. If you check the return code on PhidgetEncoder_setPositionChangeTrigger(ch, 0) prior to calling open or before it is attached, you would see EPHIDGET_NOTATTACHED.
You'll want something like (pseudo-code):
or if you have an AttachHandler put
inside of it