When using the Active Directory Migration Toolkit, you may receive an error that a user cannot be moved while they have child leaf objects. This is usually due to the user tying a device to their Exchange account via ActiveSync.
To solve this, make sure you have the Support Tools for Windows Server installed. Open up ADSI Edit (adsiedit.msc), and browse out to the user object.
Delete the phones one at a time (right-click, Delete). Once all the phone objects have been deleted, you can then delete the ExchangeActiveSyncDevices container.
Once this is done, you should have no more problems moving the user with ADMT.