A communication unit, e.g. a headset, constructed as a slave station which may be coupled to a plurality of master stations, e.g. a mobile telephone, via a short-ranged communications connection, such as of the Bluetooth type or the DECT type, and has a plurality of address fields in which one or more identification keys may be stored. At least one of the address fields has an associated electrical lock which may be unlocked only by an adapter master station having a unique indentification key. It is also possible to “lift” the receiver of the public switched telephone electrically or mechanically via the headset, so that a call and conversations from the public switched telephone may be made and take place via the headset. In an embodiment, the coupling box has a recess in which the headset may be placed and be charged via a charging unit in the coupling boy. The invention ensures that the public switched telephone is used as much as possible, which, in addition to a better sound reproduction, is favorable in terms of costs.