The w28 has a 200 ohm balance resistor in the windings, this makes the transmitter's voltage pretty limited, and you have to find a Local Battery (LB) transmitter capsule to get it working well, alternativly you have to do som soldering and add a resistor or Capacitor or even better a combination, I did so with another phone based on the same circuit:
http://www.classicrotaryphones.com/f...6439#msg226439