My iphone won't take calls, why?

I just got my iphone 4s for at&t today, called them, activated it. They did a test call. Which i received, however when i used my old phone, my home phone, and a friends phone my iphone doesn't receive a call. I'm wondering why this is so and whatdo i do? Also the phone that calls my new phone jus keeps ringing and ringig and rigig. Its annoying

Try doing a hard reset(home button and off button held together). If it still doesn't work, try setting the phone back to factory settings and try again. If that doesn't work either, call apple.

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