A fellow Berliner here. Never understood this. I live inside the A ring, yet if I am not standing close to window, even the regular call doesn't work.

More like a German thing... This happens to me in Stuttgart, too. If I want to make a call when I'm at home, I usually just call with Signal over my wifi, because the normal cellular network is terrible.

Yeah same. Doesn’t work in parts of my apartment or Treppenhaus or Hof at all, also doesn’t work in shops for some reason which can be surprisingly annoying if I quickly need to look up something for a recipe or message my girlfriend to ask if we need something. Data is also absurdly expensive.

Sounds like the building is well shielded.

My flat is the same, essentially no signal inside, just open the window and I immediately get full strength LTE.

Ask your provider for a microcell, if you want to change the situation (and often waste less battery, unless cellular is disabled smartphones tend to dislike not having signal or having poor signal quite a lot).

I’m curious if this is the location or the building; either way is relevant to me when I next move. Altbau or new?

I'm also in Berlin. I know what you mean. It became a personal joke to note that the most remote corners of the world have better internet than parts of the Ring.

Internet? We don't need that newfangled bullshit, we build the best cars in the world /s