Go through these points in order:
- Nicht stören: (Do not disturb) Is it turned on for the extension? Then it does not ring.
- Rufumleitung: (Call forwarding) Does a rule forward the call or reject it?
- Geschäftszeiten: (Business hours) Is the call outside the hours or during an exception?
- Anrufablauf: (Call flow) Does the main number in the Call builder lead to the right destination? Use Nummer prüfen (Check number).
- Gerät: (Device) Is the app connected or the desk phone assigned?
Check each change with a test call from outside.