When outside of the office, the Android app is almost never ringing. This happens since many month now. After playing around with the settings, I could achieve a more stable behavior by disabling the tunnel option. I could reproduce the following once: WLAN disabled, Tunnel on: - Call the phones extension -> Phone is ringing - Disable mobile data on the phone - Call the phones extension -> Phone is not ringing (of course) - Reenable mobile data on the phone and wait some seconds - Call the phones extension -> Phone is not ringing (terrible) The same procedure with tunnel option off has no problems. After checking this out, I was trying to reproduce it again with the tunnel option on. But unfortunately, as soon as I turn the tunnel option on, the phone cannot establish a call at all (nor in nor out). So I will disable the tunnel option by now. But it seems there is a massiv problem with it. On the server side I'm on 15.5.9348.3 Linux Debian 9. Firewall Check runs without a problem.