ISUP Redirection And Call Diversion In SS7 Networks

ISUP redirection and call diversion allow a network to move an incoming call from its original destination to another number. The process may support unconditional forwarding, busy diversion, no-reply forwarding, or redirection caused by a service platform. Although the subscriber experiences a simple transfer, the signalling network must preserve the call history and explain why the destination changed.

For Australian operators, this can involve fixed services, mobile networks, number portability, enterprise hunt groups, and services delivered across IP interconnects. Understanding the forwarding invocation procedure helps engineers interpret call traces, select suitable timers, and distinguish a legitimate diversion from a routing fault.

What Forwarding Invocation Means

Forwarding invocation begins when the called party has an active diversion service or when the originating network requests an alternative route. A service switching point evaluates the dialled number and service conditions, then creates a new call leg towards the forwarded destination. The original called number remains important because it identifies the service that triggered the change.

In ISUP, the Initial Address Message carries the core routing information. Depending on the implementation, redirection-related data can identify the original called number, the redirecting party, the reason for diversion, and how many times the call has already been redirected. This prevents a downstream switch from treating the forwarded number as the first and only destination.

ISUP Information Elements

The Redirection Information parameter records details such as diversion reason and redirection counter. Common reasons include user busy, no reply, unconditional diversion, and deflection. The Original Called Number parameter can preserve the first destination, while the Redirecting Number parameter identifies the number or service that caused the forwarding action.

The exact parameter set depends on national variants, interconnect agreements, and equipment capabilities. A trace should therefore be read alongside the applicable ISUP specification and switch configuration. Engineers working through alarm conditions may also benefit from this SS7 alarm guide, particularly when a signalling link problem is confused with an application-level redirection issue.

The Signalling Sequence

A typical sequence starts with an IAM directed to the original subscriber. The terminating exchange checks the service profile and may invoke forwarding before returning a final response. It then releases, deflects, or otherwise clears the first call leg according to the platform design while launching a new IAM towards the forwarded number.

The new IAM should carry enough history for the receiving network to understand the call context. If the destination answers, ACM and ANM progress the new call leg. If it fails, messages such as REL and RLC clear the relevant circuit or transaction. Poorly coordinated handling can create double charging, looping diversions, missing caller identity, or an incorrect busy tone.

Timers, Loops, And Failure Causes

Timers are important when the first destination rings before a no-reply diversion is invoked. A service node must wait for the configured alerting interval without holding resources indefinitely. Excessive delay can cause the originating party to abandon the call, while an aggressive timer can divert calls before the subscriber has a reasonable chance to answer.

Redirection counters and loop detection protect the network from circular forwarding. For example, number A may divert to B while B diverts back to A, or several enterprise hunt groups may eventually point to the same service. The network should stop the sequence at the permitted limit and return a meaningful cause rather than repeatedly creating new call legs.

Australian Numbering And Interconnects

Australian call flows may cross Telstra, Optus, Vodafone, wholesale, and enterprise-managed networks, with interconnection policies affecting which ISUP parameters survive. A diverted +61 mobile or geographic number can pass through several signalling domains before reaching its final destination. Number portability also means the dialled prefix alone may not reveal the current service provider.

Services in Sydney and Melbourne often involve dense carrier interconnects and hosted business platforms, while regional and remote areas can depend on longer chains of fixed, mobile, and IP transport. During the continuing transition from legacy PSTN services towards NBN-based voice arrangements, a visible call diversion may involve a media gateway or SIP application even when the SS7 trace remains at the carrier boundary.

Operational Checks For Engineers

A disciplined trace review separates the original call leg from the forwarded leg. Start with the IAM, compare the called and redirecting numbers, then follow the release and answer messages for each circuit or transaction. The SS7 Training resource provides broader material on ISUP, MTP3, SIGTRAN, and telecom architecture for this type of analysis.

Useful checks include:

• Confirm the diversion reason and redirection counter.
• Compare original, redirecting, and final called numbers.
• Check IAM, ACM, ANM, REL, and RLC timing.
• Verify charging and caller-ID treatment.

When testing a service, use controlled cases rather than a single successful call. Test an unanswered handset, a busy destination, unconditional forwarding, and a destination that rejects the call. Capture both signalling and service-platform logs so that an apparent routing error can be matched with the subscriber profile.

Practical Rules For Reliable Diversion

Forwarding behaviour should be agreed across all involved networks. A carrier may support a parameter in its domestic ISUP variant but remove it at an international gateway or SIP conversion point. Clear interconnect profiles reduce the risk that the receiving exchange interprets a forwarded call as a new, unrelated call.

Keep these principles in operational runbooks:

• Preserve the original destination where the standard permits.
• Enforce a clear maximum redirection count.
• Map failure causes consistently between call legs.
• Record diversion events for billing and fraud analysis.

Australian support teams should also account for local time zones, public holiday traffic, and business services that forward calls after hours. A Melbourne office diverting to a national contact centre may behave differently from a remote service relying on mobile coverage. The reliable design is one that keeps signalling history, resource control, and customer-facing behaviour aligned.

The key point is that call diversion is a controlled ISUP procedure, not merely a change to the destination number. Correct handling preserves the original called party, identifies the forwarding reason, limits loops, and follows every call leg through its final release. Remember that accurate redirection information is what allows the network to route the call correctly and explain its behaviour during troubleshooting.