A private number plan (PNP) is a subscriber-defined telephone numbering arrangement for a group of users. Its numbers can have a structure or meaning different from public phone numbers. The subscriber and service provider agree on the plan, and the provider configures it for the service.
What a private number plan means
ITU-T Recommendation I.255.2 defines a PNP as “a subscriber defined numbering plan which may have different structures and meanings than provided by the public numbering plan.” In practice, it lets members of a defined group use numbers assigned under that private plan to call one another.
The group may use the PNP as its only numbering arrangement or alongside other numbering, depending on the subscribed options. ITU-T says the subscriber and provider establish the plan by agreement; the provider handles provisioning and withdrawal. ITU-T Recommendation I.255.2
How users and calls fit into the plan
PNP group
A PNP group is a set of users assigned one or more numbers within the same plan so they can make and receive PNP calls.
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PNP call and virtual member
A PNP call is made by a group member to another member—or to a virtual member—using the private plan. A virtual member can receive PNP calls but cannot originate them using the plan, according to I.255.2.
Private numbers and public phone numbers
A private number identifies someone within the defined group or private telephone network; it is not automatically a complete public E.164 number. Depending on the service, the network can translate private numbers into public numbering so calls can reach users through the public network. Whether that translation and public reachability are available depends on how the service is configured. ITU-T Recommendation D.232
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Some plans may use the least significant digits of ISDN numbers as private numbers when those digits uniquely identify a group member. In SIP and corporate-network examples, a private number may be paired with a phone-context to indicate where the number is meaningful. ETSI notes that joining the private number and numeric context does not necessarily create a valid E.164 number. ETSI TR 102 634
Numbering plan versus dial plan
A numbering plan describes the private numbers and their structure. A dial plan concerns how people enter numbers and how a system interprets digits, prefixes, and routing. Providers and phone systems may use these terms differently, so check the service documentation when configuring or evaluating a particular system.
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Provider terminology and limits vary
Some carriers use the term “private dial plan” for a service that implements private numbering. For example, Verizon Business describes a trunk-based Enterprise Connection Service that can connect customer locations through a virtual phone network and support private numbers up to 32 digits; its guide says the feature is unavailable with its line-based service. These are details of that specific service, not universal PNP rules. Verizon Business Enterprise Connection Service guide
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