Recently, as I was reviewing commonly searched words and phrases having to do with cellular, I notice questions along these lines of "Do Cellular Signal Boosters Really Work?" that kept popping up.
At first I said to myself, of course they do, why would anyone be selling this stuff if it didn't work?
Then that old circuit board looking sticker came to mind. I'm sure others have seen them. They were sold as cell phone boosters. They are just a sticker that you were to place under the battery of your phone and it was supposed to boost your cell phone signal. I'm pretty sure billions of those were sold. Although I've never known anyone that used or tested them and found them to work, I have read testimonials written by who knows who that said they worked.
So, just to answer the questions (and not counting those sticker things), yes adding an external antenna directly to your cell phone will improve your signal. Adding a power booster will help even more. And, you can get very effective cellular repeater systems to improve the signal inside your home or or building.