I'm still trying to figure out how this door intercom module works. It's a 18y old siedle TLM 611-0 module, I figured out the mic part, there the audio signal is modulated onto the current the mic amp draws. But the loudspeaker part baffles me. This is the circuit diagram of the speaker amp I reverse engineered from the PCB. I don't really know how it works. It's quite a strange design and I'm not really good with analog electronics. Help is very much appreciated.
Apparently some people can't see the embedded image of the schematic, here is a link to it: https://i.imgur.com/rj3Cwcc.png
EDIT: Updated the schematic to a conventionally drawn one. Old hand drawn version of the schematic: https://i.imgur.com/Evuv3Wm.png