Internet Communications Using SIP : Delivering VoIP and Multimedia Services With Session Initiation Protocol {2Nd Ed.}

Tel URI

No Internet device needs to have knowledge of telephone numbers and their

context-specific meanings, but needs only to understand URIs such as the tel

URI [8]. The Telephony URI schema specify the “tel” name of a terminal in the

phone network as seen from the Internet, and the connection types that can be

used to connect to that entity. Telephony URIs can be used for fixed and mobile

phone calls and for fax. For example:

tel:+1-201-555-0123

points to a number in the United States, while:

tel:1234;phone-context=munich.example.com

indicates that this phone number (1234) may only be used by a SIP-proxy rec-

ognizing the phone-context (that is, the SIP server “knowing what to do”

within the domain munich.example.com).

Digit separators can be “(“, “)”, “-”, or “.” (or none). The separators are

removed by parsers and are ignored. The grouping of +1-... as the country

code for the US in the example is done for human readability.

SIP URIs can also handle telephone numbers. In fact, the entire set of para-

meters specified for the tel URI can be used in the username portion of a

SIP URI.

An example of a parameter in a tel URI is described for the phone-

context.

The phone-context

Not all tel URIs are as simple as the one shown earlier. The phone-context

specifies under what circumstances a phone number can be used. For example:

tel:1-800-123-4567;phone-context=+1-972

refers to an 800 number valid only for the North American numbers in the 972

calling area. The expression 1-800-123-4567 is called a dial string—something

used by humans.

Note the absence of a + in the URI. Any digits in a tel URI that do not begin

with a + are assumed to be a locally valid but not global valid numbers. Using

this phone number outside the calling area is not permitted and would gener-

ate an error announcement, or result in a misrouted call.

56 Chapter 4