4.18 Support of Number portability

23.2283GPPIP Multimedia Subsystem (IMS)Release 18Stage 2TS

4.18.1 Number portability

Number portability (NP) allows a user to retain their E.164 number when changing subscriptions from one network operator to another. As such, NP applies to TEL URIs and SIP URIs representing E.164 addresses. NP is subject to regional requirements and is accomplished through the retrieval of ported data from those databases. The specification of these databases is out of the scope of this document, but the NP data may be accessed through ENUM/DNS or accessed via existing (PSTN‑ and CS‑domain) NP databases using the legacy PSTN/CS-domain protocols, such as TCAP.

Support of NP within a network and the exact means to make the number portability data available to IMS, is subject to and configured per operator policy. NP is not mandated by this specification on any network operator.

As configured per operator policy, IMS ENUM interfaces can be updated to support handling of the PSTN ENUM service per IETF RFC 4769 [57], which provides a URI containing an E.164 number with NP routing information and NP dip indicators. The IMS entity receiving NP information as a result of an ENUM/DNS query (e.g. S‑CSCF), needs to support NP protocol parameters retrieved as part of ENUM/DNS procedures contained in clause 4.3.5. This IMS entity and any subsequent IMS entities/network elements used to process the call to the PSTN shall not remove the NP protocol parameters inserted in SIP messaging as part of the NP data retrieval procedure.

NP data can also be made available by means of direct access to PSTN/CS‑domain NP Databases using the legacy PSTN/CS-Domain interfaces and protocols. To support this existing interface within the network, the requesting and subsequent network elements need to support, or not remove, NP protocol parameters within SIP messages that result from the NP data retrieval procedures. The procedures to retrieve the NP data using the legacy PSTN/CS‑domain interfaces are out of scope of this specification.

Alternatively, per operator policy, the BGCF can retrieve NP data as part of the procedures to select an MGCF for PSTN connection. The interface used at the BGCF to retrieve the NP data is out of scope of this specification.

When clause 4.15a (Roaming Architecture for Voice over IMS with Local Breakout) is in use, and the Home Network decides to loop-back the call to the visited network, the Home network can choose not to retrieve NP data, and leave it to the visited network.

Alternatively, per operator policy, the MGCF may support legacy interfaces to retrieve number portability data.

NOTE: Although legacy protocols are used to access the number portability database, this does not imply that the IMS nodes (CSCFs, BGCFs) need to implement such protocols.