A telecom operator is entitled to install fibre on a façade. You cannot prevent him from doing this. He does have to meet a number of terms to do so. For instance, the contractor has to to inform you beforehand about where and how the work will be carried out. If you do not agree with this, you can then consult with the contractor. For more information, see this page.
Each operator is entitled to roll out his network until the access point of the apartment building, i.e. the location where the telecom operator’s network enters the apartment building (often the cellar or technical room) and where the operators can easily manage their connections to the various apartments. In the case of a new construction and major renovations, it is also compulsory to provide the apartment building with such an entry point.
Furthermore, each operator is entitled to access all existing in-building infrastructure (such as ducts) to connect a customer, meaning that he may use the apartment building’s common areas to this effect. This is discussed in further detail on this page.
Each operator is entitled to access all existing in-building infrastructure to connect an individual customer, meaning that he may use the apartment building’s common areas to this effect, even if the other co-owners do not agree. However, it must concern an improvement of the existing infrastructure. The procedure is discussed in further detail on this page.