Each department or office to reserve the access to only authorized personnel: students can only access it with valid reasons, or if you have an appointment with a professor, or staff. To contact them, there is a book out from the door of every closed access, with the list of numbers of those who work there and a phone. The internal numbers can also be found in the address book of the University https://rubrica.polimi.it/. In case you want to contact an internal number from a phone that’s not inside, it should be added as indicated in the contacts list, the prefix 02 2399.