Nice mpotratz!
I just followed your advice and tried the same thing. Before on December 31st, I tried connecting my obi200 device by retrying every 2-3 minutes, and would do research inbetween tries and was unsuccessful, not to mention that I made the DNS changes and swapped out the ethernet cable that came with the obi with my own thinking the cable was at fault, and in the obi firmware settings changed the timezone to match my current timezone (also experimented with keeping the phone on the line during the entire 120 second discovery process, and hanging up after hearing the message the first time, with no success. I think I made 6 connection attempts total yesterday over a 15 minute period before giving up).
However, today, I tried again, and this time just followed the instructions and would hang up immediately after the start of the first message (wouldn't even let the first message audio finish playing). I did this in succession over a 10 or 15 minute period, and was able to squeeze in 10-13 attempts during that time frame and it finally connected on the 13th try.
So, yeah, for those of you having connection issues, just retry the process without thinking into it too much or trying to do anything different than normal. It should start finally reaching the server and properly connecting. It almost seems like there's a timeout issue with their servers, and with each attempt, the device makes it further into the handshake process with the server that it's supposed to talk to. Would almost seem like expanding the discovery period on the obi site from 110 or 120 seconds to 5-10 minutes might solve the issue. But this is all me just hypothesizing without any true understanding of the network process behind the scenes. I just remember on one of my attempts (maybe around #11 or 12), I saw that my browser actually started showing that a TLS handshake was occurring, which is something I recall not seeing the first couple of times I tried. Anyways, everyone, keep on attempting those connections and don't give up, it will eventually work!