Why does she keep losing her connection?

I have a friend, who's on my cell phone's family plan & she's changed 3 phones in 6 months & still has dropped calls on a regular basis, sometimes several times in one phone call. She has U.S. Cellular as her carrier & we all live in a rural area. The nearest tower is less than a half mile away, but that doesn't seem to matter sometimes...she even gets dropped within sight of the tower! Her first phone was an old one of mine (Nokia). The second one was an LG. I can't remember what the last one was, but it had just come in the day before she got it, so it's really new (got it 2 months ago). This is frustrating for the both of us, as she can even be in my house & get a dropped call. I rarely ever get a dropped call. I've read somewhere that if you have too many fillings in your mouth, that it can interfere with your cell phone. Don't know if this is true, but that's the only thing I can think of that might be a possibility. She doesn't use a Bluetooth or headset, doesn't have texting & the battery is always charged. I've had to let her use my phone sometimes when she was dropped more than a few times on one call. She never got dropped while using my phone. I've considered taking her phone & giving her mine (switching numbers) to see how that works, as I'm out of options. If anyone has any other suggestions, I'd really be interested in hearing them. Thanks. <*)))><