I live in Central Berlin and the issue is present here as well. I had to choose the Telekom, one of the most expensive providers, because they intentionally blocked requests from other providers to service me with internet in my apartment.
I ordered service from 3 service providers and each of them called me a month after ordering and told me that Telekom is telling them there are technical problems and my bandwidth target of 50 MBit is impossible to deliver, so they aren't able to serve me.
I called Telekom after that and they told me everything is fine and aggressively tried to convert me to a customer. Tried another provider after that, still failed, so I went with...Telekom...and who would've thought...a week after that I had internet again.