What can I do against unwanted calls?
First of all, you can try to block the incoming calls from the number in question. Many recent smartphones offer the possibility to configure your phone to block numbers. You should consult the manual of your device to find out exactly how to do this.
If that doesn't provide you with the solution, you may consider contacting the Telecom Mediation Service. If you are mainly dealing with stalking or threats, the local police would be your best bet. When you file a complaint or an official record is opened with the local police, they will do what it takes to help you.
Keep in mind that both your provider and the Mediation Service cannot block individual numbers.