Nextivity Cel-fi RS1 Cell Phone Signal Booster Wireless Extender for T-Mobile To be a candidate for Cel-Fi, you must have at least one "bar" of 3G or 4G service at the location where the system is to be used. Reduces dropped calls and improves voice clarity. - Insanely Simple: plug & play, power up and Cel-Fi is set! High capacity, ideal for an office - Coverage up to 13,000 sq.ft., 60 simultaneous users and 42Mbps data. Approved and authorized by T-Mobile Only. Cel-Fi is designed and certified to improve T-Mobile 3G and 4G coverage. Advanced technology: sophisticated self organizing algorithms that maximize the coverage area.