@MatthiasK meanwhile I followed @Rose_Languste 's idea and put a resistor in between source and the Opener’s yellow wire.
But then, ringing the bell is not detected. On the other hand, falsely ringing is detected continuously, no matter if with or without the resistor continuously every ~40 seconds. Over a 2k2Ω resistor I measure a constant voltage of 1.5V, no matter if the bell is rang or not, nor if the Opener detects ringing or not.
So, I am wondering what I am doing wrong? How does the ring detection internally work? Why does it keep detecting a ring?