For some European intercoms, there is Doorman [0]. Their authors reversed engineered the protocol used by Koch and built an ESP32 + Home Assistant solution that works quite nicely (including board). The "party mode" [1] was a life saver for me when doing events on the rooftop.
https://www.ziritione.org -- Started in the early ~2000s. Personal blog, sometimes too personal (I added a bunch of noindex tags instead of removing old content), covering some tech stuff over the years. Interestingly, I migrated the site to different frameworks over the years, still managing to keep most of the URLs stable and not losing any meaningful content (started with blosxom in perl, then custom solution in django and now hugo).
[0] https://doorman.azon.ai/ [1] https://doorman.azon.ai/guide/features/ring-to-open