Business internal dialing refers to the dial structure within a single business entity, and is sometimes referred to as Virtual Private Numbering (VPN) or Private Numbering Plan (PNP).

This structure allows users a shorter number sequence for numbers within the same building or company (or in theory, a whole of any size, the only real restrictions come from regulations).

CIBC in Toronto

Toronto

Toronto is the provincial capital of Ontario and the largest city in Canada. It is located in Southern Ontario on the northwestern shore of Lake Ontario. A relatively modern city, Toronto's history dates back to the late-18th century, when its land was first purchased by the British monarchy from...

 used 6-XXXX (XXXX denotes the last four numbers of the full 10 digit number) for numbers originating from 416-784-XXXX numbers and 7-XXXX for numbers 416-780-XXXX. All external numbers used 9-416-XXX-XXXX sequence. This sequence is part of the Nortel Networks Meridian 1 Communications Systems and Meridian Mail system.

This sort of system is fairly common in different types of PBX Systems, with these being a few examples:

• Merlin Legend Mail by Avaya
  Avaya
  Avaya Inc. is a privately held computer networking, information technology and telecommunications company that is a global provider of business communications systems. The international head quarters is in Basking Ridge, New Jersey, United States...
  
  
• Trio Mobile Office by Trio AB

Unrelated to this term is the concept of a Virtual Private Number.