Although the iPhone 3G and newer uses your cellular provider's 3G network to transmit data to and from the device, your phone may experience connectivity issues that causes it to use the EDGE network instead. According to Apple, testing out your 3G signal by testing the connection in a different location can correct many issues related to the iPhone's network. If your iPhone continues to experience connectivity issues, you can turn off the 3G feature in the phone's network settings, which controls how the phone connects to your cellular provider.
Click "Settings" on the iPhone screen and click "General."
Tap "Network" and tap the switch on the "Enable 3G" tab. Tap the "Cellular Data" switch to turn off the feature.
Tap the switch on "Enable 3G" and on "Cellular Data" to re-activate the data network. The "3G" icon will appear on the home screen once it reconnects to the network, according to Apple.