Method for operating a system for controlling access to a restricted access zone in a building or a site, wherein the system comprises a transmitting and receiving device, a storage device (26, 28) containing a database in which a plurality of user profiles of users authorized to access are stored, a processor, and an image processing device (16, 22, 24) which has a camera located at the entrance to the restricted access zone, the method comprising: receiving, by the transmitting and receiving device, a device-specific identifier of a mobile electronic device of a user when the mobile electronic device is within radio range of the transmitting and receiving device in a public zone from which the user present in the public zone can request access to the restricted access zone; storing the received identifier of the mobile electronic device in a further database of the storage device (26, 28) as belonging to a user present, wherein the further database is designed to store identifiers of mobile electronic devices currently present in the public zone, wherein such a mobile electronic device is assigned to a user registered or not registered in the access control system and wherein the identifier of a mobile electronic device assigned to a registered user is assigned to a user profile of the registered user stored in the database of the storage device (26, 28); generating, from a camera recording of the user present generated by the camera of the image processing device (16, 22, 24), a real-time template for facial features of a user present when a presence of the user present is recognized in a detection range of the camera when the user present requests access to the restricted access zone, wherein the real-time template is generated by the image processing device (16, 22, 24); and searching the user profiles stored in the database for a reference template that matches the real-time template to a specified degree, wherein a user profile is searched only if it is assigned to a user present based on a received identifier stored in the further database, wherein, in case of a match, the system grants the user access to the restricted access zone and denies access if there is no match. receiving, by the transmitting and receiving device, a device-specific identifier of a mobile electronic device of a user when the mobile electronic device is within radio range of the transmitting and receiving device in a public zone from which the user present in the public zone can request access to the restricted access zone; storing the received identifier of the mobile electronic device in a further database of the storage device (26, 28) as belonging to a user present, wherein the further database is designed to store identifiers of mobile electronic devices currently present in the public zone, wherein such a mobile electronic device is assigned to a user registered or not registered in the access control system and wherein the identifier of a mobile electronic device assigned to a registered user is assigned to a user profile of the registered user stored in the database of the storage device (26, 28); generating, from a camera recording of the user present generated by the camera of the image processing device (16, 22, 24), a real-time template for facial features of a user present when a presence of the user present is recognized in a detection range of the camera when the user present requests access to the restricted access zone, wherein the real-time template is generated by the image processing device (16, 22, 24); and searching the user profiles stored in the database for a reference template that matches the real-time template to a specified degree, wherein a user profile is searched only if it is assigned to a user present based on a received identifier stored in the further database, wherein, in case of a match, the system grants the user access to the restricted access zone and denies access if there is no match.