Check the appstore that your provider doesn't have an iPhone app. I'm with Vodafone in the UK and they have an app which gives you information about your usage.
If they don't have an app then the information can be seen in the settings app under General->Usage->Cellular Usage. If you reset it monthly then you can see how much you've used. It includes stats for cellular & tether data
shortcut(ish): prefs:root=General&path=USAGE