CEL-FI meets the newer regulatory requirements that allow a booster to be used (boosters that do not meet these requirements, which is the vast majority of them, are no longer legal).  For the FCC you can learn more by reading the new FCC Report and Order for signal boosters. According to these new regulations, consumers must receive permission from their carrier before using a booster. Consumer boosters sold after March 1, 2014, and some before that date will be marked with a label signifying it meets the FCC’s new regulations.