The SIP protocol as defined by RFC 3261 makes no provision for conveying much of the information associated with a call on SS7 networks. In particular information about Local Number Portability is not included.
SIP Number Portability Parameters
RFC 4694 - Number Portability Parameters for the "tel" URI - provides a way to add some of this information to the SIP call URI.
The most common added fields are probably npdi and rn.
npdi indicates that a query to the NPDB (Number Portability Data Base) has been done and that the call should be routed using the phone number in the rn field (if present) instead of the original called number.
Here is an example:
INVITE sip:8085551212;rn=8087321000;email@example.com:5060 SIP/2.0
Breaking it down:
- 8085551212 is the original called number
- rn=8087321000 is the number that should be used for routing the call - also known as the LRN — in this example, this is not the number that the call should be delivered to, it is only to be used for determining how to route the call.
- npdi=yes indicates a lookup in the number portability data base has been done
If the number porting database has been checked, and the number is not ported then the URI would have have npdi=yes and the rn field would be omitted:
INVITE sip:8085551212;firstname.lastname@example.org:5060 SIP/2.0
These fields often seem to be present when exchanging calls with a Sonus switch or gateway.
Created by: admin, Last modification: Mon 20 of Dec, 2010 (19:37 UTC)
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