If a phone-contexttag is not present in the URI, then some other context
(typically geographic) must be used to interpret the digits. Since the Internet
does not have the kind of geographic isolation typically present in the PSTN,
this is a difficult thing to do.
NOTEIn the PSTN, a Class 5 telephone switch in a North American Rate
Center covered by only a single numbering plan area code (NPA) may safely
assume that any 7 digit number uses the default area code and country code.
This is possible because of the structure of the PSTN and the limited
connectivity that this switch has to other switches in the PSTN. In a SIP network
on the Internet, a proxy is theoretically accessible to any SIP user agent in the
world, and such assumptions about global validity of a phone number must not
be made.SIP URI
SIP URIs [9] used are very similar to e-mail addresses within SIP messages to
indicate the originator (From) and the final recipient (To). SIP also has several
new headers such as current destination (Request-URI), intended recipient (To),
and the direct route address (Contact), among others. This will be explained
in more detail in Chapter 5, “Real-Time Internet Multimedia.”
When used with a hyperlink, the SIP URI indicates the use of the INVITE
method. SIP URI hyperlinks allow the embedding of links that when opened
can initiate a phone call, for example:
sip:[email protected]may be used to send a call to a voice mailbox. Or:
sip:[email protected];user=phoneindicates how to address a call from the Internet to the PSTN E.164 phone
number +1-214-555-1212 via the IP telephony gateway having the domain
name gateway.com. The user=phonetag is a hint to parsers that a tele-
phone number is present in the username portion of the URI and is not just a
numerical name.
The host portion of a SIP URI containing a telephone number does not
always indicate a gateway. This is because the creator of the URL may not
know the location of the gateway and may instead be relying on a proxy to
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