4 iphones later and still goes direct ...

Hi
My iPhone 3G has problem where calls, not all, but suddenly go direct to voicemail. Full signal, and in different locations. I can then not call out - just long silence then beep beep beep. I am on 02. If I restart the phone it often does clear the issue but I only know I am missing calls when someone calls me on my spare phone!
I have now been through 4 iPhones, two sim cards and even closed down the account and started again with new number and it still does it. I have to carry two phones so people can call me - it is ridiculous and 02 can not find any faults.
Help!!!!

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