Do you have an Exchange account setup on the iPhone?
Passcode Policies can be pushed from the Exchange server to the iPhone. If this were the case, you would have to contact the Exchange administrator to have the policy changed.
"Policies can be set on iPhone and iPod touch in one of two ways. If the device is
configured to access a Microsoft Exchange account, the Exchange ActiveSync policies
are wirelessly pushed to the device. This allows you to enforce and update the policies
without any action by the user. For information about EAS policies, see “Supported
Exchange ActiveSync Policies” on page 6."
http://support.apple.com/manuals/enUS/Enterprise_DeploymentGuide.pdf
Jason