on o2 pay as you go you used to dial 4444 but on pay monthly you have to either call o2 customer services on 202 or check on the o2 website with your login details x
O2 have just launched a new service to get your remaining allowances etc by SMS or WAP
All you need to do is send a blank message to 21202 for free and they will reply with an SMS with the details.
If your handset will not allow you to send a blank SMS then enter any character, it doesn't make a difference.
The SMS will also contain a WAP URL which will go to the same information online. As long as you always use that URL then it will not cost you anything to view this information. If you try to access the information by any other method then standard browsing charges will apply.
NB: If you are on an older billing system or on a business account then you may receive an SMS saying this service is not available. All normal consumer pay monthly accounts are in the process of being moved to the new billing system so if the service is not yet available to you then wait until you receive a letter from O2 stating they are moving it over for you. The service will not be available on accounts with more than 1 mobile phone number registered against them either.