In addition to configuring the traditional phone server and enabling extension mobility on the device, for the DX650 one additional setting must be enabled on the device level.
1) In Cisco Unified Communications Manager Administration, select Device > Phone.
2) Find a Cisco DX600 Series phone to configure for extension mobility multi-user.
3) In the Product Specific Configuration Layout portion of the Device Configuration window for a Cisco DX600 Series phone, select the Enabled value for the Multi-User drop-down list box.
4) Save and Apply Config.
Once Multi-User has been enabled you can access EM from Settings > Personal > Extension Mobility or from the lock screen "Sign in as New User" in the top left.