I've been battling with O2 for months trying to get them to acknowledge there is a problem! I've done all of what you listed Tonyb1968, sadly they either won't pass it to the Network team, as they say it's been going on too long to be a fault, not enough people are complaining to them about it, or they've sent an engineer out who hasn't found a problem, well clearly there is!
I've just tried my phone from a landline, three times out of five it would not ring, with five full bars of signal, O2's response is still, try another phone and SIM card replacement, I'm on phone number three and SIM card number three, and it makes no difference!
I don't think anything is broken enough for them to see it as a specific fault in my case, I think it's just stretched to capacity, so when tested shows it is working. It's what I believe is a case of under investment in the network being overtaken by the amount of smartphones sold on the network and data usage exceeding their expectations and network capacity.
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