Hi Hannes,
let’s make the following assumption.
Your Installation use 3 wire ERT | 1 | 7 check Installation Page 26ff.
HTS/711-…
1 Bezugsklemme für Sprechen, Rufen, Öffnen
7 Rufklemme und Versorgung
ERT Etagenruftaste (gegen Klemme +)
15/16 Einschaltung des Monitors → you don’t have
This mean that the white cable is responsible for the communication. Maybe Siedle is using a bus-protokoll which mean the Nuki is “cancelling” this protocol due to their device and no intercom is working only read the request.
If we look to the Colour code of Nuki
So I believe that the orange cable would work if we would have an “Analog Mic” I could not find any documentation of the green cable, but I guess the App told you to use.
Mean you could try to connect with an additional wire the two original white wires to PIN 1 again and if the Nuki still receive the signal from PIN 7 you may have change intercom works again and Nuki can open, if the nuki is not working you have to live without intercom.