4 iphones later and still goes direct to voicemail

Question

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!!!!

Answer 1

I'd love this lady's name at O2, thanks. I was offered 2 weeks free service by one guy, but I nearly spat in his face...
I'm at a loss of what to do - I love the iPhone and everything else which isn't O2 related (voice calls, SMS, voicemail) seems to work perfectly. It's interesting that you're East London too - can we narrow it down further? I live in Limehouse and work in Shoreditch. It's worst in Shoreditch, but also happens in Limehouse (as it is at the moment).
I do also have the long wait to phone out problem sometimes. I also get dropped calls more than I should. And text messages sometimes take an age to send. But the most annoying thing is the 9 times out of 10 that someone tries to call me that it just goes through to voicemail. This is compounded by the length of time it takes voicemail and SMSs to get delivered to me.
Btw, this is the other, similar thread:
http://discussions.apple.com/thread.jspa?threadID=1952845&tstart=15
And to answer all the obvious questions people ask:
• yes, have tried soft reset
• yes, have tried hard reset
• yes, have tried reboot (helps initially straight after, but 5 mins later issues return)
• yes, have tried resetting to factory settings
• yes, have tried setting it up as a completely new iPhone (using iTunes)
• yes, have disabled call forwarding (did again seem to work better for a while, but issues return)
• yes, have replaced iPhone
• yes, have replaced SIM card
I haven't tried the 3G switch off - but as Stanley says, that renders all the good reasons for having an iPhone almost redundant.
Very, very annoying.