I have the same problem, has been going on for over a week now, can't view bills and can't keep track of the kids' mobile usage. Just get "service unavailable", what's the problem?
What can happen is that if you have a number of accounts listed, the page can select the wrong account number, so make sure you select your main account from the dropdown list.
This is more likely to happen if you had older account numbers which are no longer active.