Why is my AT&T iPhone 4 service so bad?

I have had my iPhone 4 for well over two years and lately I get absolutely no service wherever I go, is this a battery problem or what? My mom has the same phone she hasn't had it for as long but her iPhone 4 and her boyfriends iphone 4 work just fine. My service is just bad. And I need to figure out why.

Call AT&T costumer care they will help really.

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