Virtual Numbers are direct inward dialing numbers that are not tied to a particular seat. There are two types, both of which are set up the same way.
Virtual Numbers are found under Enterprise Services in the Customer Administration Portal, in a section called Virtual/Toll-Free Numbers.
Company Main Number: every PBX has a Company Main Number, which can be edited to point to any group extension (such as a hunt group, and auto attendant, or a call center). Press the Edit button on the right side of the listing to assign it from the drop-down menu.
Virtual Number: a true Virtual Number incurs a recurring monthly charge, but can be pointed toward any extension (even a seat) in the PBX. Press the Edit button on the right side of the listing to assign it from the drop-down menu.