There are several reasons to perform a hard reset on an iPhone 3G. The phone may crash after a software update, refuse to synchronize with iTunes or the screen may freeze, making it impossible to make or receive a phone call. A hard reset repairs most hardware and software issues and helps the phone perform to full capacity; however, Apple support suggests only resetting the iPhone when turning it off and back on does not solve the issue.
Simultaneously press and hold the "Sleep/Wake" button on the top edge of the phone and the round "Home" button on the bottom of the phone.
Ignore the "Slide to power off" option when it appears on the screen of the phone. Continue to press and hold the "Sleep/Wake" and "Home" buttons.
Wait for the Apple logo to appear and release the "Sleep/Wake" and "Home" buttons. The phone is now in the reboot/reset process. When the Home screen or "Enter Passcode" appears, the reset process is complete.