When a phone is in the login mode, it uses a particular user device profile. When a phone is in the logout mode, it uses the default device profile. If both profiles are configured with the same directory number, the user device profile takes precedence over the default device profile when the directory number is dialed.

Configuring Extension Mobility on CME:

http://www.cisco.com/en/US/docs/voice_ip_comm/cucme/admin/configuration/guide/cmemobl.html