12.6. The tel URI
The telephone URI (tel URI) is used to identify resources using a telephone number. SIP allows requests to be sent to a tel URI. This means that the request-URI of a SIP request can contain a tel URI.
The tel URI can contain a global number or a local number. A global number follows the rules of E.164 numbers and starts with a "+", while a local number follows the rules of local private numbering plans. Local numbers need to have the phone-context parameter, which identifies the context (owner) of the local number and, therefore, the scope of the number. This makes the number globally unique. The context can be represented by a global number or a domain name: the former must contain a valid global number that is owned by the local number distributor, and the latter must contain a valid domain name that is under the authority of the owner distributing the local numbers. Here are some tel URI examples:
a global number – tel:+358-9-123-45678
a local number with a domain context – tel:45678;phone-context=example.com
a local number with a global number context: tel:45678;phone-context=+358-9-123
Notice that the tel URI allows visual separators like hyphens "-" in the number to improve readability and the tel URI parameters are separated by semicolons ";". The full tel URI syntax can be found in [RFC3966].
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