ISUP diversion data: location and original called numbers
Call diversion changes more than the destination of a telephone call. It can also change the signalling information carried between exchanges, softswitches, and gateways. In an ISDN User Part (ISUP) network, the Original Called Number and Location Number help downstream equipment understand what happened before the call reached its current destination.
These parameters matter when a subscriber forwards calls to a mobile, voicemail platform, call centre, or another service number. The final called party may answer successfully while the signalling record still needs to preserve the first number dialled and the relevant serving location.
The distinction is easy to miss because carriers use different national profiles and may apply screening, privacy, or number-formatting rules. A useful analysis therefore considers the Initial Address Message (IAM), redirection indicators, numbering plans, and the service provider’s implementation notes together.
This is particularly relevant in Australia, where calls may cross legacy PSTN exchanges, SIP interconnects, mobile networks, and NBN-based voice services. A call placed in Brisbane, for example, may be diverted to a Sydney contact centre while retaining information needed for billing, troubleshooting, and lawful network operations.
What the original called number represents
The Original Called Number (OCN) identifies the destination that the caller initially requested before diversion occurred. If a customer dials a fixed service and that service forwards calls to a mobile, the mobile number becomes the current destination, while the OCN preserves the original service number.
This is different from the calling party number. The calling party number identifies who originated the call, subject to presentation restrictions. The OCN identifies the first called endpoint. Applications can use it to select an announcement, apply service logic, associate the call with the correct business line, or produce an accurate call-detail record.
The OCN is especially valuable when several customer-facing numbers terminate on one platform. Without it, a call centre may see only the forwarded destination and lose the information needed to identify which queue, department, or published number the caller used.
How the location number fits diversion
The ISUP Location Number parameter generally supplies a network or service location associated with the call or called party. It is a numbering identity, not a live geographic position. It should not be interpreted as GPS data or as proof that the person answering is physically in a particular suburb.
In diversion scenarios, the location number may help an exchange or service platform identify the relevant access location, serving area, or routing context. Its exact use depends on the national ISUP profile, carrier agreement, and switch vendor. Some networks populate it consistently; others omit it, translate it, or carry equivalent information through another signalling mechanism.
Australian engineers should check the carrier’s interconnect specification rather than assume that an 02, 03, 07, or 08 number always describes the current physical endpoint. Number portability, hosted voice, and national call-centre operations can separate a number’s geographic appearance from the equipment actually handling the call.
Reading the signalling sequence
A typical diverted call begins with an IAM containing the called party number. Redirection information can then indicate that forwarding or rerouting has taken place. The OCN may preserve the original dialled number, while the called party number in a later leg identifies the diversion target.
The redirecting number, where supplied, usually identifies the party or service that caused the redirection. It should not be confused with the OCN. For example, the original number may belong to a shop in Geelong, the redirecting identity may be the shop’s forwarding service, and the final called number may belong to an employee’s mobile.
When investigating a trace, compare the IAMs on each call leg. Check nature-of-address indicators, numbering-plan indicators, odd/even address signals, and translation between national and international formats. A +61 representation and a domestic 0-prefix representation may refer to the same Australian number while appearing different in raw captures.
Common implementation and privacy issues
A provider may suppress the OCN because of privacy policy, inter-carrier limitations, or a switch configuration that does not support the required parameter. The field may also be rewritten during transit. This can create misleading records in voicemail, fraud controls, or customer-service applications.
Presentation restrictions deserve careful handling. A network should not expose restricted calling information simply because an OCN or redirecting number is available internally. Operational logs, SIP headers, and ISUP fields may follow different screening rules, so access controls must cover every conversion point.
Training material from SS7 Training can help engineers relate these fields to MTP3 routing, timers, ISUP message structure, and SIGTRAN transport. In a mixed environment, the important question is where the information is created, translated, and consumed—not merely whether a field appears in one packet capture.
Comparing the key identities
The following simplified example shows how the identities can differ during a diverted call from an Australian business number to a mobile service.
| Signalling item | Example value | Meaning during diversion |
|---|---|---|
| Calling party number | 03 9123 4400 | Number of the person who placed the call |
| Original called number | 07 3344 2200 | Number dialled before forwarding |
| Redirecting number | 07 3344 2200 or service identity | Party or service that initiated diversion, depending on profile |
| Final called number | 04 1234 5678 | Mobile or other endpoint receiving the diverted leg |
| Location number | Carrier-defined location identity | Network or service location context, not GPS position |
The values above are illustrative. A carrier might encode the OCN and location number in different messages, omit one field, or use a national variant. A trace must therefore be read against the relevant Australian interconnect documentation and the switch’s parameter mapping.
It is also wise to keep protocol analysis separate from unrelated web material. A Dutch casino reference may sit on the same domain, but it does not define ISUP behaviour or substitute for a carrier signalling specification.
A practical troubleshooting method
Begin with the customer-facing number and record every diversion rule, including time-of-day routing, voicemail overflow, and simultaneous ringing. Then capture the signalling on each available leg and mark the called, original called, redirecting, and location identities separately.
Next, compare the trace with the service provider’s parameter definitions. Confirm whether the OCN survives an SS7-to-SIP conversion, whether the location number is translated at a gateway, and whether national formatting changes the value. Test both an answered call and an unanswered or busy diversion, since different service paths can produce different parameters.
For a concrete next step, capture one diverted call from an Australian fixed number to a mobile and annotate each IAM field against the carrier’s ISUP profile.
