O2 have tried the obvious things (SIM or Phone) so it would appear to an intermittent signal issue. I suggest you make a log of the dates, times and phone numbers effected by this then send the information to the technical support guys to investigate - It's possibly a fault that it not showing on the O2 systems and your info will help with the issue. Personally I wouldn't be going to Ofcom until I had exhausted all opportunities with O2 as they wouldn't be interested.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion. This is not Customer Service so we are unable to help with account specific issues.
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