The integration created that “Call monitor phone book” sensor which was not changing state. I did already removed and added again but to no avail.
Now I removed everything, rebooted and saw in the states that the attributes get created for this phone book sensor AND the state is changing.
It was very confusing that the sensor was named Call monitor phone book (and not related to the state of the line) whereas before this was just Call monitor so I thought this was something new because of the different/newer OS on the device.
Update:
The problem is not solved.
It worked but now, without any change, the sensor is not changing state.