When i used to send an SMS it would show my remaining balance on the screen with a 'Dismiss' button... and now i dont get that screen after sending a message anymore 😕😕
*10# and then press "call" will present your credit balance on the screen.
You used to have to ask CS to switch the balance confirmation off but it annoyed many people, perhaps the new default is now off and you need to ask CS if you want it on.
My 3GS on O2 also stopped showing the balance yesterday after calls and texts. Both my girlfriend's 3G and dad's 3GS stopped too, so I can only assume it's something O2 have done to the network.