Hi Willi,
A phototransistor has a "dark current" which is its leakage when dark. So it is already conducting and doesn't have a threshold. Adding some base current with a biasing resistor to the positive supply will only increase its idle current without increasing its sensitivity to light.
Using a resistor from its base to emitter will decrease its sensitivity because more light is required to create a voltage drop across the resistor, causing a threshold before the phototransistor begins to conduct.