ISUP Call Setup: IAM, ACM, CPG and ANM Walkthrough
ISUP, or ISDN User Part, is the application-layer protocol that allows telephone switches to exchange call-control information on SS7. Rather than carrying speech itself, it establishes a media path, reports progress and eventually releases the connection. A typical setup sequence begins with IAM and continues through ACM, CPG and ANM, although optional messages can appear between them.
This article walks through ISUP call setup with IAM, ACM, CPG and ANM as one end-to-end transaction. It focuses on the fields engineers encounter in packet captures: message type, direction, circuit identification code, routing labels, number information and cause values, with Australian deployment details included where they affect interpretation.
ISUP's place in SS7 call routing
ISUP operates over MTP3, which routes signalling messages between signalling points. MTP3 uses the Signalling Link Selection and a Circuit Identification Code, or CIC, to identify a speech circuit. ISUP supplies the call semantics that MTP3 does not: which number to dial, whether a route exists, whether the call is progressing and whether the called party has answered.
In a traditional TDM network, this signalling logic sits beside a timeslot-based media path. In converged infrastructure, the same ISUP dialogue may run over IP through SIGTRAN. The SIGTRAN adaptation guide explains how M3UA, M2UA, M2PA and SUA carry or adapt SS7 traffic while keeping many ISUP procedures recognisable.
IAM starts the call setup transaction
The Initial Address Message opens the transaction. The originating exchange populates it with the called party number, often a Calling Party Number (CPC), and other switching information. It also includes routing data used to select a downstream signalling point and, normally, a CIC for the new speech circuit.
IAM can cross carrier and network boundaries, so its number fields and labels must reflect the Australian numbering plan along with any translation or portability lookup applied upstream. The exchange is not waiting for an answer at this stage: IAM asks the downstream network to locate the destination and accept the call.
Analysts should distinguish an ISUP message from its lower-layer delivery acknowledgement. Successful MTP3 transfer confirms reliable signalling delivery, not that the destination exchange has found a usable route.
ACM confirms the downstream route
The Address Complete Message responds at the ISUP level by indicating that the downstream address has been interpreted and a route is available. ACM carries transaction and circuit identifiers and can return backward-route information for use by the originating exchange.
ACM therefore means address complete, not answered. A trace ending at ACM may show successful routing but no off-hook event. Repeated setup messages can sometimes reflect rerouting or an alternate attempt, so engineers should compare the CIC and transaction context rather than count messages in isolation.
CPG carries progress before answer
Call Proceeding tells the originating side to continue toward ringing or answer. It normally travels backward after ACM and can include a cause value such as Busy, Congestion or No Information Available. If the destination cannot continue, it can send a final Release message and terminate the attempt.
CPG may also carry optional progress information, but it remains distinct from an Alerting indication or Ringback tone. In Australian traces, engineers should also check whether early media is present, since it may come from an announcement service rather than the called party.
Comparing the four setup messages
Each message answers a different question about the call and contributes information needed for the next stage.
| Message | Main role | Common information | Typical direction |
|---|---|---|---|
| IAM | Requests a route to the called party | Called number, CPC, CIC, routing data | Originating to downstream |
| ACM | Confirms that an address and route are available | Circuit identifiers, backward-route data | Downstream to originating |
| CPG | Allows setup to continue | Optional cause or progress data | Downstream to originating |
| ANM | Reports that the called party answered | Answer supervision, optional information | Terminating to originating |
Direction can appear reversed at gateways because each exchange describes the route from its own perspective. The reliable method is to follow the CIC, switching point and route labels across the complete transaction.
ANM marks the answered state
The Answer Message is generated when the called party answers. ANM travels back along the established route, usually after any optional Alerting or Ringback indication, and marks the transition from setup to answered supervision. A traditional switch can then connect the originating and terminating speech paths, subject to media-resource control and any early-media arrangements.
ANM closes the setup phase but not the call itself. A later Release, followed by a Release Complete response, ends normal call supervision. The operational distinction is straightforward: ACM validates addressing, CPG permits continued progress, and ANM confirms an actual answer.
Practical checks and Australian context
ISUP traces remain relevant in Australia where legacy switches, PSTN interconnects and carrier gateways coexist with IP-based infrastructure. Telstra, Optus, Vodafone and other providers operate in a market shaped by the NBN, mobile number portability and widespread mobile use. Network teams in Sydney and Melbourne regularly examine cross-provider and international traffic, while fixed-line services remain important for regional areas and businesses.
Australian callers routinely use 04xx mobile numbers alongside 1300 and 1800 service numbers. Each category follows ACMA numbering rules, and portability can prevent the destination exchange from identifying the customer's home carrier directly. Emergency traffic for 000 must be identified separately because it follows special operational arrangements.
Operational handling also falls within the Telecommunications Act 1997, the Telecommunications (Numbering) Act 1997 and the Privacy Act 1988. Telecommunications operators and designated critical-infrastructure entities may have further duties under the Critical Infrastructure Security Act 2018.
Message-reading checks
- Match the IAM and ACM identifiers and CPC before following a particular call leg.
- Interpret direction alongside point codes and routing labels, especially at gateways.
- Separate MTP3 transport acknowledgements from ISUP response messages.
Administrators preparing protected training files can also use this PDF security comparison to review document-access options for authorised material.
Australian trace checks
- Validate 04xx, 1300, 1800 and geographic number formats against current numbering rules.
- Flag portability lookups and cross-carrier handoffs in the routing sequence.
- Identify 000 traffic separately and apply its emergency-handling controls.
- Protect retained calling-party information and access to sensitive signalling logs.
The lasting lesson is that IAM requests a route, ACM confirms the address, CPG reports that progress may continue and ANM records the answer. Accurate ISUP analysis depends on reading each message, direction, CIC and cause together within the network and regulatory environment that produced the trace.
