A first list and a second list of identifications are established, with which a telemarketing advertisement or no undesirable telemarketing advertisement is expected. The identification of the caller an incoming call is compared to the identifications in the list. If the identification of the call is not found in the lists, the call is routed to a call processing which enables the reception of a telemarketing advertisement to ascertained and, based on the result of that determination, the lists are correspondingly updated. The calls for which the identification of the caller is recorded in the second list are routed to the called party and the calls for which the identification of the caller is recorded in the first list are rejected. Such protection against telemarketing is particularly efficient in the context of an industrial operation with numerous human resources and for lines on which several calls can be simultaneously carried.