not insert sim but unregistered sim when I lose signal for some time. It's a crap shoot however as sometimes with both my N73 and E61 it corrects itself and others i'm forced to do a reboot. Either way it's a real pain in the *** sometimes!!!
Is this an old SIM by any chance? Or one of the new Cingular ones? I have an old ATTWS one..which has worked without problems in my old SE phone (T637) but I get this error once in a while with the E61.