Does anyone have experience with cell phone boosters? My sister just got into a new apartment and her phone does not work in her unit. So, I'm thinking that maybe if she get a booster, it might help the signal? Do cell phone boosters need to have an antennae or something? Because that might not be possible. Or can they just be plugged in a sit on a table or something? The carrier is AT&T, if that matters. What type of cost would I be looking at?