There have been quite a few voice and data monitoring apps over the years but none have stuck around for long. The latest is Telicost which claims to measure and provide alerts and alarms for your data, voice, and SMS usage. I am not sure how they monitor data usage since RIM butchered the API that provides those specific details in OS 5 but maybe they found a way around that.
The Telicost app is totally free so it wont cost you a penny to give it a spin. The app also integrates with their Intelicost enterprise server which can consolidate and report on all corporate devices from one place.
Reviews of the app seem to be mixed but all in all I have found it to be pretty good so far at least for minutes and SMS usage. Check out Telicost at this link in App World and let us know what you think of the app!