This compact surface-mounting unit in aluminium has a surface-mounting height of only 16 mm 16 mm. The external unit is equipped with a 5 mm-thick name plate in Plexiglas which is vandal proof. The metal-plated push-button contacts are maintenance-free and durable. This external unit works with 2-wire technology, which means that an existing bell can simply be replaced by this external unit. This external unit has 2 bells.