Even when my cell is on & not busy when others call it goes straight to voicemail. How do I change this?

I have a blackberry curve and it is with AT%26amp;T.Even when my cell is on %26amp; not busy when others call it goes straight to voicemail. How do I change this?You either don't have service, battery power, or have call forwarding set up. Call up at%26amp;t.Even when my cell is on %26amp; not busy when others call it goes straight to voicemail. How do I change this?You don't have service.