Siemens Gigaset charger interface (A130 / A220)?

Anyone knows what the (old) Siemens Gigaset DECT handsets require from the charging cradle?

I would like to convert a A220 base to a charger cradle - but can’t quite figure out what the handset needs. Seems there’s a simple transistor circuit and a 22R series resistor in the base. Might be controlled - or might just be a short circuit protection? Have no supply for the unit - so I’m not even sure of the voltage levels… Anyone has a clue? Or a schematics (ie. of the charger cradle, the base station, or the handset)?