Is my iPhone cheating me with data usage? How can I check it? I reset my data usage on my iPhone after asking this question, so at 28th of July. Since then I was using my phone all the time, watching YouTube, talking on Skype (both voice and text), downloading games from the App Store, using Facebook (including uploading videos and photos) and finally sending emails with attachments (photos, each photo around 1MB). 

After 6 days of using my phone intensively, I checked data usage. It shows... 411KB sent and 2,3MB received. 

I know it is not right - but how can I check real usage?
Were you on wifi? I think the usage only tracks 3G. If you are looking for your 3G only data, try ATT's account app. It will tell you exactly what your data usage is according to your ATT account.