I'd contact customer service, but even then you may have some work to do, as it's very rare so most if not all customer services haven't experienced it and will probably tell you to try a new SIM card/ disconnect & reconnect your number.
It basically means that there is an error on the O2 network relating to your number, where changes on your number (like activating a new SIM card) aren't activated fully on the O2 network. The full name for it is an SOG error, but whether you can find anyone who knows what this means....
The customer services should check the SIM card registered for your number anyway, but if this doesn't fix it, the best thing to do is to ask the customer services to report this to a department called Central Migrations, who can check things behind the scenes. Hope this helps
