I don’t think there is any such law, not that I could find. Telekom refuse to make the connection. They say that if it isn’t there when the apartment is built a new one can’t be added. I live in an attic conversion
I just checked.
There is no right to a land-line but a general right to being connected.
So somebody has to provide you a connection.
If nobody does you can message the "Bundesnetzagentur" and they will oblige a provider.
I recommend to check the law here :)