[haggisman]

Grateful for advice. Out of contract with Orange, my wife and I ported our respective numbers to two new iPhones, purchased at an Apple store.
I've had a seamless transition. Sadly she has not. Number ported across without difficulty. However, her phone intermittently receives no calls. When calling it there is the usual delay while the system searches for it, then voicemail kicks in. Other times call gets through. This occurs when no data transfer is simultaneously occurring. She can make calls, send and receive test messages. O2 have tried a variety of system reconfigurations/hard resets etc with intermittent and short lived success. Culminated in advice to have Sim checked at an O2 shop, duly done, nil found, but a new sim given her to be activated if Apple store found nil amiss. So, she duly attended Apple store (in worsening humour, having had a belly full of hanging on for phone tech support, journey to O2 shop etc) After much faffing about, a genius bar tech simply swapped phone.
All seemed well. Today, nil changed, same problem. Settings on her phone as factory, except for home wifi being enabled. She has full 3g coverage at work (more than I have). Fault has occurred in a number of locations, not just one cell. How likely is a Sim fault, despite O2 shop assurances that it was ok? Any ideas what else to do? My long term assurances that Apple stuff will work/ be replaced until it does wearing very thin now.

[haggisman]

Follow up to own post; SIM change seems to have resolved the issue - despite checks in O2 shop that the original one was ok.
Not sure if might be a solution for some of the signal strength issues folk have flagged up. No prior experience with O2, but 15+ years with Orange and a nuhost of handsets used by the clan have never seen a SIM problem.
Just a thought , know nothing of SIMS really.