The 850-894 MHz signal is the standard CDMA frequency band. In the US, Sprint, Verizon and US Cellular use CDMA. CDMA carriers use network-based white lists to verify their subscribers. That means you can only switch phones with your carrier's permission, and a carrier doesn't have to accept any particular phone onto its network. CDMA is a common 2G signal frequency, which is mainly used to block SMS signals and can effectively solve students' text messages in class.
Cell Phone Jammers By Bands