ISUP Pre-Answer Setup and Cause Code 20 Handling

In an SS7 call, the period between the Initial Address Message (IAM) and answer is where much of the network’s signalling logic takes place. The destination may be ringing, temporarily unavailable, redirected, or unreachable, yet the calling party should receive a clear result rather than silence or an unexplained release.

Pre-answer handling matters in Australian fixed, mobile, and IP interconnects because calls commonly cross several signalling domains. A call from a Melbourne business on an NBN voice service may pass through a carrier softswitch, a SIGTRAN link, an international gateway, and an older PSTN exchange before reaching the subscriber.

What Pre-Answer Means In ISUP

The originating exchange sends an IAM containing the called number, calling party information, nature of connection indicators, and transmission requirements. The receiving side may then return an Address Complete Message (ACM), indicating that routing is complete and the call is progressing, without confirming that the called party has answered.

A Call Progress Message (CPG) can provide additional progress information, such as alerting or in-band announcements. The call remains in a pre-answer state until an Answer Message (ANM) is received. Billing and answer supervision should generally begin at ANM, not at IAM or ACM, unless a specific interconnect arrangement defines another treatment.

Understanding Cause Code 20

Cause value 20 is commonly described as “Subscriber absent”. It indicates that the network cannot locate or reach the intended subscriber at the time of the call. The reason may involve a handset being detached, a mobile device being out of coverage, a service state in which the user is unavailable, or a translation and routing condition that maps to this Q.850 cause.

The code should not be confused with a busy condition, which is usually represented by cause 17, or with network congestion, commonly associated with cause 34. Accurate interpretation is important because the announcement, retry behaviour, charging treatment, and SIP response mapping can differ for each result.

Building The Signalling Sequence

A typical sequence begins with IAM, followed by ACM when the terminating exchange has accepted the call for further processing. If the destination is available and alerting, the network may send CPG and provide ringback to the caller. If the subscriber cannot be found before answer, the terminating side should release the call with a suitable REL message carrying cause 20.

The exchange that receives REL must stop alerting, clear any reserved resources, and return an appropriate release response where required by the signalling profile. A Release Complete (RLC) confirms that circuit or bearer resources have been freed. In an IP-based environment, the same state transition may be carried over M3UA and SCTP within SIGTRAN rather than across a traditional 64 kbit/s voice circuit.

Timers And State Control

ISUP timers protect the network when a response is delayed or lost. T7, for example, supervises the expected response after IAM, while other timers govern address completion, call progress, answer, and release procedures. Exact names, values, and actions depend on the national profile and carrier implementation.

A timer expiry should not automatically be labelled as subscriber absent. If the network has no reliable terminating result, it may need a general failure, no-route, or congestion cause instead. Engineers should record the message sequence, point codes, circuit identification code, timer event, and SCCP or MTP3 transport status before selecting a replacement cause.

Mapping Cause 20 Across SIP

When an ISUP gateway maps cause 20 into SIP, the selected response should preserve the meaning of subscriber unavailability as closely as possible. Implementations may use a 4xx or 5xx response depending on the gateway vendor, carrier policy, and whether the condition is treated as a user, service, or network failure. The mapping should be documented rather than assumed from a generic cause table.

The reverse direction also needs care. A SIP 480 Temporarily Unavailable may be translated to ISUP cause 20 when the destination is known but currently absent. A 404 Not Found may instead suggest that the number or user cannot be located. Poor mapping can produce misleading announcements, repeated retries, or a release that appears successful to one network while failing operationally in another.

Australian Interconnection Considerations

Australian carriers operate across large geographic areas, from dense Sydney and Brisbane exchanges to remote communities where mobile coverage and backhaul availability vary significantly. A transient coverage gap should not be treated in the same way as an invalid number. Monitoring should compare cause 20 rates by carrier, access type, state, and time of day.

The migration from legacy copper services to NBN voice platforms has also increased the importance of gateway testing. Retail providers may use SIP internally while presenting ISUP towards another carrier. Interoperability tests should cover Australian numbering formats, national and international prefixes, CLI privacy, emergency service routing, and calls involving mobile and fixed networks.

Supplementary Services And Failure Paths

Call forwarding, call transfer, diversion, and connected-line services can change the destination after the initial IAM. A forward-transfer procedure may trigger another signalling exchange, so the original call leg must remain consistent with the new route and its timers. The forward transfer guide is useful when studying how supplementary-service signalling affects call progression.

Australian operators must also consider privacy obligations under the Privacy Act 1988 and operational expectations set by the Australian Communications and Media Authority. Trace files should minimise unnecessary personal data while retaining enough signalling detail to investigate failed calls. Emergency calls require separate operational treatment and should never be routed or retried solely according to an ordinary cause-20 policy.

Testing should include an absent mobile, a powered-off endpoint, an unreachable NBN voice service, a forwarded number whose second leg fails, and a delayed ANM. Each test should verify the caller’s audible treatment, release timing, billing start point, gateway mapping, and final resource clearance.

A reliable implementation treats cause 20 as a precise pre-answer outcome, not a convenient label for every unsuccessful call. The essential record is the full message sequence: IAM, any ACM or CPG, the point at which the destination becomes unavailable, the REL containing cause 20, and the final RLC. That sequence is what lets Australian SS7 and SIGTRAN engineers distinguish subscriber absence from routing, timer, transport, and interworking faults.