Give a local number its own context
03A local extension needs a context that identifies its numbering scope. A context is not a prefix to concatenate or a host to contact.
In the RFC-style example tel:863-1234;phone-context=+1-914-555, the numeric context labels a scope. It is not an algorithm that turns the local number into a valid global number.
Key idea: The local number and its context together identify the number.
Terms
- tel URI
- A Uniform Resource Identifier for a telephone number. tel names the telephone scheme; it is not an instruction to dial.
- Global number
- A number written with a leading + and its international country and national number.
- Local number / context
- A number meaningful within an identified scope, such as a private phone system. The context distinguishes otherwise identical local numbers.
- phone-context
- Required for a local number. A controlled domain or a suitably assigned global-number prefix labels the numbering scope.
Rules & examples
The local number and its context together identify the number.
- A local number MUST have a phone-context parameter, chosen to identify the scope in which that number is unambiguous. Local form SHOULD NOT be used when the number can be expressed globally.
- A domain-name context MUST be under the administrative control of the entity managing the context. It does not have to resolve to an actual host. No DNS lookup, SIP host discovery or domain-ownership proof is implied by merely reading the URI.
- A numeric context can be a global number or suitable leading digits whose entire matching global-number range belongs to the same organization. If no exclusive prefix exists, the organization SHOULD use its lowest assigned global number. Verified erratum 202 corrects the example of an unsuitable shared prefix to +1-212-555-01.
- The context does NOT supply digits that can reliably be prepended to make a global number. A recipient intending to call a local number MUST understand the context and be able to place calls within it; simply storing it as an identifier does not require that dialing knowledge.
Fictional offline examples only: no dialer is opened and no call is placed. URI syntax does not verify number assignment, reachability, ownership or caller identity. Selected examples are not a complete telephone or SIP implementation.
Sources
New to this? Start hereA quick picture + helpful background
A number is not a dialing procedure
Think of a contact card: it names who to reach, while the phone system supplies the steps for making a connection. A tel URI similarly identifies a number; its context and parameters help interpret it but do not authorize a call or promise reachability. This is an analogy, not a telephone-routing algorithm.
Helpful first, if these ideas are unfamiliar
These pictures are analogies to help you learn. The linked RFC explains the actual rules.
Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.