The Treos used to come with a bookmarked link to the Sprint PCS Vision mobile site. It looked like a WAP site, and you could navigate the links by pressing various numbers on your keypad to access the detail of your account, including the breakdown of minutes used.
This site now is only about managing content, ie, buying more services like ringtones and games. You could call *4, but the recording says that it won't be current for the last 24 hours. However, by asking for more info about my usage, the voice said that 8 minutes of my 50 bonus minutes that Sprint is giving now were used.
The best would be an SMS service for checking usage. I have this with my banks, although I wouldn't use this for doing any transaction considering the mobile worms that are cropping up.