Because these jammers in particular prevent all communication by mobile telephone, they can represent a danger, since emergency calls are rendered impossible. Most of the time, advertising for this kind of device is misleading: it implies that their use is legal and takes care not to indicate that even their possession is illegal.
Initially expensive, bulky and limited to blocking mobile communications in the GSM 900 MHz frequency band, jammers have mirrored technological development. They have become smaller, cheaper and can now block much more of the spectrum. Indeed, in addition to the mobile telephony bands (GSM, UMTS, LTE, ...), some models allow jamming of devices of navigation satellite systems tracking (GPS, Glonass, Galileo, ...), data transmission (WLAN, RLAN, WiMax, ...) or wireless alarms.
Further information from the Federal Office of Communications OFCOM