The use of the called party number in ISUP IAM routing
In ISUP, the Initial Address Message (IAM) carries the information an originating exchange needs to establish a circuit-switched call. Among its parameters, the Called Party Number (CdPN) is central to route selection because it identifies the destination selected by the caller.
The receiving switch analyses the digits, numbering plan, nature of address, and related indicators before choosing an outgoing trunk or service route. This process connects traditional PSTN signalling with modern SS7 and SIGTRAN environments, where the signalling path may use IP while the call-control logic remains closely aligned with ISUP practice.
Australian networks provide useful examples. A number beginning with 04 generally indicates a mobile service, while geographic numbers use area codes such as 02 for New South Wales and the Australian Capital Territory, 03 for Victoria and Tasmania, 07 for Queensland, and 08 for South Australia, Western Australia and the Northern Territory. Service ranges such as 13, 1300 and 1800 require different treatment from ordinary subscriber numbers.
| Routing situation | CdPN analysis | Typical result |
|---|---|---|
| Local geographic call | Area code and subscriber digits are assessed | Local or regional route selected |
| National long-distance call | National destination format is recognised | Inter-exchange trunk chosen |
| Mobile call | Mobile prefix, commonly 04, is identified | Mobile carrier route selected |
| 13, 1300 or 1800 service | Special service range is matched | Service-platform or intelligent route used |
| International call | International access and country code are processed | International gateway selected |
How CdPN appears in the IAM
The CdPN parameter contains address signals accompanied by fields that explain how those signals should be interpreted. The Nature of Address Indicator can identify a subscriber, national significant number, international number, or another numbering context. The Numbering Plan Indicator commonly identifies the telephone numbering plan being used.
The address digits may be encoded in semi-octets, with an odd/even indicator showing whether the final digit occupies a complete or partial digit position. A switch must decode these details correctly before applying its digit-analysis rules. A malformed indicator or an unexpected digit count can lead to rejection, misrouting, or an IAM being sent towards the wrong trunk group.
Digit analysis and route selection
A transit exchange usually compares the CdPN against ordered translation tables. These tables may match an exact number, a prefix, a range, or a service code. The longest or most specific valid match normally takes precedence, allowing a mobile range or premium service range to override a broad national rule.
Routing logic can also remove, add, or replace digits before the next IAM is transmitted. For example, an exchange may receive a national number, remove the national trunk prefix, and forward the appropriate significant digits. When numbering portability is involved, the initial prefix may lead to a database lookup rather than directly identifying the current carrier.
Australian operators must account for the mix of geographic, mobile and non-geographic services found in markets such as Sydney, Melbourne and Brisbane. A route designed only around geographic area codes will fail to handle 13 or 1800 services properly, while a rule that treats every 04 number as permanently belonging to one carrier may become inaccurate after number portability.
CdPN and other IAM parameters
CdPN analysis does not operate in isolation. The Calling Party Number can influence screening, presentation and permitted return routes, while the Forward Call Indicators reveal capabilities such as continuity checking, interworking and end-to-end signalling conditions. The Transmission Medium Requirement helps determine whether the requested bearer service can be supported on the selected path.
The circuit identification code, point codes and routing label determine where the IAM is delivered within the SS7 network, but they do not replace the destination analysis performed on CdPN. A signalling point may receive a correctly addressed message and still select an unsuitable circuit route if its numbering translation is wrong.
The Network Indicator also matters when signalling traffic crosses network boundaries or uses different national and international contexts. Engineers working through these cases can refer to the network indicator field to relate routing decisions to the SS7 network context.
Number portability and service translation
Ported numbers make CdPN routing more dynamic. The dialled number remains stable for the customer, but a portability database may return a network destination, routing number, or carrier identifier. The originating exchange may perform the lookup, or a later service-control platform may do so before the call reaches the terminating network.
Intelligent Network services add another layer. A 1300 business number might be translated to a state-based contact centre, while an 1800 number could be directed to a national platform. These translations should preserve the correct called-party information and ensure that subsequent exchanges receive a number format they understand.
The same principle applies when calls move between legacy SS7 and SIP-based infrastructure. An interworking function maps the ISUP CdPN to a SIP Request-URI or related telephone-event information, then restores compatible signalling when required. Differences in international prefixes, national prefixes and presentation formats must be controlled at the boundary.
SIGTRAN transport and operational faults
SIGTRAN carries SS7 user parts such as ISUP over IP through protocols including SCTP, M2PA or M3UA. This changes the transport path without changing the basic purpose of CdPN. A message can arrive through an IP signalling link while route selection still depends on ISUP digit analysis and destination tables.
Faults may arise from either layer. An SCTP association can be healthy while an M3UA routing key, point-code configuration or application server state is wrong. Conversely, the signalling route can be operational while a digit translation sends calls to an invalid destination. Monitoring should therefore correlate IAM traces with link status, point-code reachability and call outcomes.
Useful trace fields include the raw CdPN, decoded nature of address, numbering plan, routing label, originating and destination point codes, circuit identification code, release cause and any portability lookup result. Comparing a successful call from Perth with a failed call to the same service from Melbourne can reveal whether the fault is geographic translation, carrier interconnection or service-platform routing.
Testing a reliable IAM route
A practical test plan should cover local, national, mobile, international and special-service numbers. In Australia, test cases might include a Sydney geographic number, a Melbourne number, an 04 mobile, a 13 service and an 1800 service. Each test should confirm the digits placed in the outgoing IAM, the selected trunk group and the final release or answer result.
Engineers should also test odd-length and even-length numbers, national and international formats, unavailable portability responses, and calls crossing an SS7-to-SIP gateway. Packet captures and signalling monitors should be compared with switch translations so that an apparent routing fault is not confused with a codec, bearer or termination issue.
Where documentation or escalation is needed, the contact team can be a suitable point for training-related queries. The essential operational habit is simple: decode the CdPN exactly as received, verify its numbering context, follow the translation rule that matched it, and confirm that the resulting IAM is suitable for the next network element.
