the obvious next step is to recognize the bell ringing by the shelly.

as it is done by rise of voltage level, I had something like this in mind, but as I have no experience I'm not sure if it will work....

(I figured it out from other projects on the internet using an ESP on other similar intercoms :

https://forum.iobroker.net/topic/7660/siedle-klingel-t%C3%BCr%C3%B6ffner-mit-esp8266-und-mqtt

)