Phonecalls cut every 30 sec Phonecalls go to Voicemail iPhone 3G

What is wrong with this thing?
EACH AND EVERY PHONE CALL is cut every 30 sec and the message Call Failed is displayed. THAT HAPPENS ON FULL COVERAGE AND 3G. It happens on any coverage and regardless of the data mode.
The other thing is that after I restart it I start receive all the missed call messages and after I accept a call and it fails after 30 sec it goes back to redirecting every call to voicemail. It stays like that until I restart it.
I AM GETTING SICK OF THIS THING BECAUSE I OWN MY BUSINESS AND THIS IS MY ONLY ADVERTISED NUMBER!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! A CUSTOMER CALLED ME JUST NOW 7 TIMES UNTIL WE FINALLY SWITCHED TO THE LAND LINE BECAUSE WE COULDNT TALK.
AS WE SPEAK I CALL FROM MY WIFES IPHONE TO MINE AND IT GOES TO VOICEMAIL WITHOUT RINGING. BOTH HAVE THE SAME COVERAGE LEVEL (FULL).
SOMEONE FROM APPLE GET IN CONTACT WITH ME SOMEHOW. CALL ME, EMAIL ME DO SOMETHING BECAUSE THIS IS DRIVING ME INSANE. IF I LOOSE MORE BUSINESS I WILL HAVE TO SUE SOMEONE.
This is an iPhone 3G, on the Australian network Optus.
Firmware Version: 3.1.2 (7D11)
Carrier: Optus 5.0
Serial Number: 84833KTSY7H
Model: MB489X
You could have called me your biggest fan 1 year ago, I recommended this phone to hundreds of my customers and many bought it, but now I am loosing faith in this.
Ovidiu Radu
Predaking Windows XP Pro