@Thank you very much for your great help. I tried out your new node_helper.js file. Unfortunately, it still didn’t work.
I continued working with your changes and discovered the issue: I am running the very latest version of Node.js (v24). However, the node-fritzbox-callmonitor module is ancient. In the background, it utilizes a network interface that—in modern Node versions—sometimes listens to thin air if the IP family (IPv4 vs. IPv6) is not explicitly specified. It ends up silently connecting to the Fritz!Box’s IPv6 address while the incoming signal arrives via IPv4—meaning it completely misses the call notification.
Additionally, I changed the IP address in the configuration file from 192.168.1.1 to fritz.box. It seems to be working now; my blocked number is no longer being displayed. I will continue to monitor the situation and provide feedback if anything changes. Thank you very much for your work.