I have a Wilson Sleek for my Verizon cell phone. Very happy with it. Having said that, I have been in places like Langtry Tx where there was no signal. Don't know if AT&T has a signal there either. Not much of anything there except Judge Roy Bean's West of the Pecos Courthouse and Saloon (the real original one). No cell or TV at all - nothing! If there is no RF signal, your booster will not create one and you will not have service.
It will only work with one cell phone at a time. You can use it with you MiFi Hot Spot, which will allow up to 10 Wifi users.