This has been bugging me for ages, because I know it can be done.
I had thought that you could dial *101#
But that doesnt seem to work any more
*100#, I believe.
"Service Not Available"
On prepay *100# checks your credit balance. They may not work after a SIM replacement for up to 24hours. If it's not working contact O2 and have them check your account.
On billpay it's *101# to check your unbilled calls balance ex VAT. Then it's also *111# to check your remaining mins / texts / data. Neither of these services work proper within your first month on bill or within 24 hours of a SIM replacement. Note that this service is now a discontinued service so may only work intermittantly or maybe not at all. They may also not work on some legacy price plans and can give inaccurate readings if you add / remove addons or change price plan mid cycle.
Check out our FAQ entry: http://www.boards.ie/vbulletin/showpost.php?p=59807260&postcount=32
Sorry, I presumed that you were prepay.