Some users report significant data usage due to unknown causes, or seemingly excessive usage when performing tasks such as checking e-mail (one user reports the transfer of more than 1 MB of data even when no new messages are received).
A number of users report inexplicable, massive data transfers in the middle of the night, when the phone is not in use, and Push and automatic e-mail checking are turned off. Interestingly, several users have reported that the data transfer occurs at 2:30 AM EST (11:30 PST)–even when Wi-Fi is on (it appears that the iPhone may deactivative Wi-Fi during standby in some cases to save power).
To monitor your iPhone’s data usage, navigate to Settings > General > Usage. If you use AT&T, you can also check your usage for the month by dialing *3282# and pressing call, or download the free AT&T myWireless app for more detailed usage.
Turn off cellular data A obvious but effective method to avoid unwanted data usage is to simply turn off cellular data when the phone is not in use. To do so, go to Settins > General > Network and turn slide cellular data to off. Some users find it especially useful to toggle this setting off overnight.
Turn off 3G As a less-drastic fix, some users have reported that simply turning off 3G (switching to EDGE-only) results in elimination of the mysteriously large data transfers. To do so, go to Settings > Network and slide Enable 3G to off.
Delete then re-establish e-mail account Some users have reported that deleting then re-establishing some e-mail accounts can result in a dramatic reduction in data usage. To do so, navigate to Settings > Mail, Contacts and Calendars and select the desired account, then press the red delete button. Next, tap the “Add Account” button and reenter your account information.
Reset warnings Speculation holds that some instances of this issue may be due to transfer of diagnostic or other information. Try resetting warnings as described in this Apple Knowledge Base article, or opting out of iAds data collection for targeted ads and check for reduced data usage.