There are no APIs that allow an iOS app to fetch this information. However, since you're writing an enterprise app, mobile device management (MDM) servers can query managed devices for device attributes such as the serial number using the DeviceInformation MDM command.
Once your MDM server has that data, the MDM server can use Managed App Configuration to push a configuration dictionary to your app. The app can then inspect that dictionary and use that information as required.
Please see DeviceInformation on page 40 and Managed App Configuration and Feedback starting on page 69 of MDM Protocol Reference for details. There is also the Managed App Configuration sample code demonstrating how this works.
If you are a member of the Apple Developer Enterprise Program, you should be able to see MDM Protocol Reference located at https://developer.apple.com/downloads/index.action?name=Mobile%20Device.
If you are not able to see the documentation as described above, please contact the Apple Developer Program support team at <https://developer.apple.com/contact/submit.php>.