ISUP call diversion: forwarding with or without redirection numbers
ISUP call diversion is the SS7 control used when a switching network sends a call somewhere other than the number originally dialled. It supports services such as unconditional, busy and no-answer forwarding. The key signalling distinction is whether the diversion information also carries a redirection, or redirecting, number.
Australian engineers commonly use “call diversion” and “invoke forwarding” for the network procedure, while customer documents usually call the feature “call forwarding”. The choice affects downstream identity presentation and fault analysis rather than simply changing the ringing number. A landline in Melbourne forwarded to a mobile in Sydney demonstrates why these terms must be separated.
What call diversion carries
The Call Diversion information element is commonly carried during Initial Address Message (IAM) setup. It states the diversion reason and may include an optional Redirection Number value, often called the redirecting number. This differs from the called-party information that establishes where the call is ultimately delivered.
Without a redirecting number, the downstream exchange retains the reason for diversion but lacks an explicit source identity for the forwarding subscription. With one, it can identify the number associated with that diversion, subject to presentation and screening rules. Neither value should automatically be confused with the rerouted destination.
How forwarding is invoked
Invoking forwarding is the control-plane action that activates a stored diversion rule. In mobile networks it may begin as a MAP InvokeForwarding operation over TCAP; ISUP then describes the resulting route with Call Diversion information. The switching point consults subscription data, often through a service control function, before issuing the routed IAM.
Message-level study is useful because the service request, subscription lookup and network setup are separate events. The SS7 training resources support practice with IAM decoding, timers and the surrounding ISUP procedures.
Unconditional forwarding can be applied before destination alerting. Busy and no-answer forwarding depend on a response or timer at the original destination, so the diversion may only become visible later in the dialogue. Operators can differ in whether they add diversion data to setup or to a subsequent message.
Forwarding without a redirection number
Without a redirecting number, the network still conveys that forwarding is active and why it was applied. The receiving exchange has no explicit diversion-source value to associate with the forwarded call, although other signalling elements may still show the original caller or called number.
This form can suit networks that minimise identity disclosure, but it limits peer-level diagnosis. A destination may know that the call was diverted yet be unable to identify the line on which forwarding was invoked. Any caller-display result then depends on local presentation policy and information retained elsewhere.
Forwarding with a redirection number
With a redirection number, the diversion information includes the identity associated with the forwarding service. The value can support attribution, troubleshooting and destination-side handling, but presentation restrictions may still prevent it from appearing on a handset or in an analytics record exposed to users.
The number is not automatically the new destination and should not be treated as the caller’s physical location. It may belong to a shared PBX, virtual service or forwarding platform. Accurate interpretation therefore depends on the subscription model used by the Australian carrier.
Australian interconnect realities
Australia’s ACMA-controlled numbering plan makes this distinction particularly visible. Landlines use geographic area codes, mobiles commonly begin with 04, and business services use numbers such as 1300 and 1800. International records normally use +61 and omit the domestic trunk zero, so a Melbourne-to-Sydney diversion can cross both geographic and formatting boundaries.
The market combines major carriers such as Telstra and Optus with wholesale and retail providers, each handling forwarding through its own service platform. NBN services are IP-based, yet SS7 and SIP interconnects may coexist at carrier boundaries, so ISUP behaviour can still surface in traces involving Australian voice traffic.
Emergency routing adds a firm boundary: 000 calls must not be diverted because doing so could delay response or obscure the caller’s location. Forwarding design also matters where a council exposes a 1300 line for smart parking or emergency-access services share infrastructure. Guidance on public safety continuity can be found in public safety resources.
Comparing the two signalling behaviours
A compact comparison shows where the two forms diverge:
| Aspect | Without redirection number | With redirection number |
|---|---|---|
| Diversion reason | Carried in diversion information | Carried in diversion information |
| Source identity | No explicit redirecting-number value | Redirecting-number value present |
| Caller display | Falls back to other available presentation fields | May expose the diversion source if permitted |
| Operational use | Reduces disclosure but limits attribution | Supports attribution and destination handling |
| Main risk | Loss of forwarding-source context | Accidental disclosure or identity misassociation |
These are protocol-level expectations, not guaranteed display rules. Carrier screening, presentation restrictions and downstream platform support can further change what an agent sees, so decoded messages and field evidence remain decisive.
Safe deployment and verification
Implementation should verify the service intent before changing signalling. For unconditional forwarding, confirm that the IAM reflects the active rule. For busy or no-answer cases, capture the relevant response, release or reason information and check timer handling before attributing the route to the destination exchange.
Trace numbers in Australian display form and international form, including +61 conversion, and distinguish the rerouted destination from the redirecting identity. Test voice, data and recording platforms separately because one may consume the diversion information while another only displays a caller label.
The practical takeaway is to verify both the forwarding reason and whether a redirecting number is present, then confirm the carrier’s presentation and screening rules before accepting the diverted route.
