To interfere with your phone, all you need is a device that transmits at the correct frequency. Although different cellular systems process signals differently, all cellular networks use radio signals that can be interrupted. GSM is used in digital cellular and PCS-based systems and operates in the 900 MHz and 1800 MHz bands in Europe and Asia and the 1900 MHz band (sometimes referred to as 1.9 GHz) in the United States). The GSM jammers can transmit on any frequency and is effective for AMPS, CDMA, TDMA, GSM, PCS, DCS, iDEN and Nextel systems.
The degree of jammer effects can vary depending on a number of factors, such as distance from the tower, indoor and outdoor environments, the presence of buildings and landscapes, and even temperature and humidity. There is concern that a rough-designed signal jammer will interfere with the functioning of medical devices such as pacemakers. However, like mobile phones, the most commonly used devices operate at a low enough power (<1W) to avoid problems
Cell phone interference laws vary globally. For example, in the UK and Japan, anyone can own a gsm jammer-as long as it is not used. Dozens of countries, including Canada, Mexico, New Zealand, Norway, Turkey and others, allow police or prison officials to use cell phone jammers. Schools in China and India use jammers to stop fraudsters. Mexico allows gsm jammers in churches and hospitals. Pakistan allows intervention in banks and libraries.