Dialable Number.
This is a phone number alias for a FirstClass Unified Communications  user’s Mailbox (the user’s extension or virtual extension). Every FirstClass Unified Communications user has a DN, or multiple DNs, which can include pager, cellular, or home phone numbers.
This is any phone number or extention that can be dialed and exists to Voice Services. It includes numbers that resolve to physical phones, as well as virtual or phantom numbers.