When you set up an iPhone, iPad or iPod Touch that is enrolled, you see the following screen during setup:
During device activation, iOS activation servers provide the device with the URL of the MDM server. This URL is assigned by an organization's IT administrator through the MDM Server DEP management portal. The Setup Assistant screens users see might vary depending on the configuration settings applied to the device by the MDM server. Contact your MDM vendor for more information about how to configure MDM enrollment settings during activation.
Once a device has been activated, it is not possible to 'redo' the activation unless the device is restored to factory settings. See Use iTunes to restore your iOS device to factory settings for more information on restoring an iOS device.