The key function of a passive off-air system is to intercept GSM phones (incoming as well as outgoing calls). The system monitors passively the air interface and therefore has no influence on transmitted numbers. The called party will always see the original calling number. Depending on the type of encryption on the air interface (5.0, 5.1, 5.2), such systems can deliver excellent results. If 5.1 encryption is used, the key must be known, if not, then pure passive systems will not work with systems currently on the market. Passive off-air systems are portable, and their range can be quite substantial (several kilometres) in combination with the use of directional antennas.