In cases of harassment by telephone, please report the offence to the police. The criminal investigation authorities will take care of everything else, i.e. obtaining a court order, making enquiries to the network provider, etc. In this way, the caller can be identified even if he has suppressed the telephone number.

You can prevent callers who bother you from being put through to you. Please log in to the customer portal. You can define which caller is to be blocked via “Phone functions”  > “Blocking list”. If the phone number is suppressed, please enter “anonymous” in the “Phone number” field. The lock then applies to all callers who call you with number suppression.

Here you can find what to do with blocking lists in detail.