Cell phone jammer also block emergency calls, such as those that may be made to doctors and emergency personnel. For example, it can prevent residents of houses from calling emergency 911. According to US telecommunications regulations, this is why its use is illegal. The FCC (Federal Communications Commission) believes that it blocks public airwaves and violates the right of individuals to use telephones. It also shields medical equipment, such as pacemakers used by heart patients. If you find that you are using the device, you will need to pay a fine.
If used for the right purpose, cell phone jammers are definitely a boon. Regardless of the nature of the surrounding environment, the speaker who has been talking on the phone can finally keep quiet.
The shielded signal transmits the high-power signal of the same frequency as the mobile phone, so that the signals conflict and cancel each other. This is the basic working principle of any shielding device. Simpler devices only block one set of frequencies. High-end equipment can block all frequencies at the same time. Cell phones are full-duplex devices, which means they use two different frequencies; one for listening and the other for talking. Some basic shields can only block one of these frequencies. However, this has the same effect as preventing them. This is because the mobile phone assumes that there is no service available because it can only access one of the frequencies, which is not enough for its normal operation.