My Iphone 4 can't make or receive phone calls

My Iphone can't make phone calls or receive them.

I have full signal, tried adjusting the SIM card, rebooted the phone several times... nothing worked.

When i call my phone from a different line, it rings once and goes to voicemail.

When I attempt to make a call, the phone never rings back until it automatically ends the call for me.

HELP!

-Ben