Gotta agree w ith UConn GB - gotta disagree with Boba.
At my business, there was ZERO signal in the ground floor. Couldn't send a text or make a call, with any carrier, ever. I put in a 200 dollar Chinese booster and now there are 3 bars, and various carriers work well. This one had an omni directional antenna on the roof. Signal on the street outside was 1 to 2 bars. The roof antenna is up about 40 feet off the ground, and no obstructions. The building is thick brick, with neighboring thick brick buildings. Coverage area inside is about 20' by about 60 feet.
At my wife's business, she had 1 bar outside on a good day. Maybe 2, 3G dicey. We put in a 70 dollar Chinese booster and now, in a very small space - maybe 15x15, there is reliably 2 bars and 3G. This has a Yagi directional on the roof, up only about 15-20 feet. Critically, you must install the antenna in the correct plane and then point it at the nearest tower. Also, do not kink the lead or put sharp bend in it (learned this the hard way). Negative Amazon reviews on these things are usually from people who install them incorrectly. They work. Definitely. Get the repeater and antenna a good distance apart, point the antenna (if directional), and you'll get results IF, and ONLY IF, there is a signal outside of the building.
PM me if you want to know the exact models I bought. BTW - installation is simple, but follow the basic rules (separation, no tight bends), and don't expect the very cheap models to give you too much coverage inside.