On my phone in adress book I go to top up credit (4444) then choose option 2 from the automated voice, then option 2 again and it tells you how many messages you have left on your allowance then how many minutes you've got left.
i'm guessing like me u are an old Tmobile user just changed to o2?? i've called 02 with the same question, apparentley there isn't a number to call. i have to check my balance online. so much easier on T mobile.