It's a tricky one, basically it means that your SIM isn't registered to your mobile number somewhere on the O2 network, and usually happens following a transfer from one department to another (i.e. business to personal). It's tricky mainly because it's very rare so customer services generally haven't heard of it! This means you'll probably get the standard info like trying wiping the SIM, SIM in different phone, new SIM, even disconnecting and reconnecting the number.
I think the technical name for it is an SOG error, which is where different levels of the network have conflicting information. Best solution for this (once basic checks like SIM wipe is for customer services to ring through to a department called Central Migrations who can check this
