I just got through doing a mountain of work on this item.
I found a web site that has great information on antennas and the like, but ... I ordered from them and their stuff didn't work. And then I ordered stuff to fix the first stuff, which also didn't work. Their "shipping and handling" charges were freaky expensive and hen they charged me a 20% restocking fee when I returned it all. And of course I had to pay for the postage to send it back to them. The moral of the story is: DO NOT BUY CELL PHONE ANTENNAS ONLINE!
I found two stores in town that sell this sort of thing. The stuff I got from them advertised "13db gain!" and the small print said something about how you get this gain if the antenna is mounted high and outside and you normally use your phone inside. The reality is that that antenna gets you about 7db gain. My cell phone would get 0 or 1 "bars" inside. 1 or 2 bars outside. With this antenna, I get 3 or 4 bars.
Let your fingers do the walking to find a local provider. All of the cell phones have goofy antenna adapters and that will be your biggest challenge. Calling around a bit got me from $140 to $90. That's for the antenna (with cable), the adapter and a mount for the roof of the house.
DO NOT BUY THIS STUFF ONLINE! I love to shop on-line, but this stuff is just way too easy to screw up. And I got extra burned, twice.