5110 SIM registration failed

Archived from groups: alt.cellular.nokia (More info?)

My Pre-pay SIM has been deactivated because I didn't make a chargeable call
for 6 months (I know - what a stupid thing to let happen). O2 helpdesk is
the most unhelpful helpdesk I have ever had the dubious pleasure of
contacting - however.....
I have another mobile phone - also a Pre-pay on O2 (or cellnet as it was) -
which works fine.
I want to swap the SIM into my 5110 so that I can still use the in car kit
etc - but the other SIM card still comes up with the same message in my 5110
even though it works fine in my other phone.
My Nokia says it's not locked to either SIM (#4) or to the network (#1)
Is there something obvious that I've missed or is my 5110 completely useless
now?
After days of trying to get advice from the O2 helpdesk I've completely
given up with them - specially as all the credit that I had put on the card
seems to belong to them now!
Any help/advice would be appreciated
Thanks