Q.850 cause 2 No route to specified transit network (national use)
Your call carried a request to route via a particular transit carrier, and the switch handling it does not recognise that carrier. It is a routing-instruction problem, not a problem with the number itself. Rare in the UK now, and normally traced to a legacy indirect-access prefix or a withdrawn interconnect.
What Q.850 cause 2 No route to specified transit network (national use) actually means
Some call setups name the transit network they want to be carried over, rather than leaving the choice to the originating operator. This cause says the equipment received such a request and either does not know that transit network at all, or knows it but has no interconnect with it. Note that the destination number is never examined — the call is refused before it gets that far, which is what separates this from cause 3. RFC 3398 maps it to SIP 404 Not Found, the same status as several other routing causes.
This cause indicates that the equipment sending this cause has received a request to route the call through a particular transit network which it does not recognize. The equipment sending this cause does not recognize the transit network either because the transit network does not exist or because that particular transit network, while it does exist, does not serve the equipment which is sending this cause. This cause is supported on a network-dependent basis.
— ITU-T Q.850 section 6.2.7.1.2
The SIP equivalent of Q.850 cause 2
A gateway translating this cause into SIP reports it as SIP 404 Not Found. The mapping is defined in RFC 3398 section 7.2.4.1.
What causes Q.850 cause 2 on a business phone system
- A legacy indirect-access code in the dial planCarrier access codes were used to force calls over a chosen operator for cheaper rates. Where those arrangements have long since lapsed, a rule left in an old PBX still prepends the code and the modern carrier has no idea what it refers to.
- An interconnect that has been withdrawnTransit relationships between operators are commercial and change. A route that was valid last year may no longer be served, and the switch will refuse a request naming it rather than silently picking a different path.
- A transit selection field copied across a gatewayWhen a gateway translates between ISUP and SIP it may carry over a transit network selection parameter that meant something on the old side and nothing on the new one. The next switch rejects the instruction it cannot interpret.
- A routing label misconfigured on an interconnectOn wholesale interconnects the carrier identity is configured at both ends. If one side is given the wrong identifier during a change, calls that reference it come back with this cause until the configuration is corrected.
How to fix Q.850 cause 2
- Strip carrier access prefixes from your dial planSearch the outbound rules for anything prepending digits ahead of the national number. On a modern SIP trunk the carrier chooses the route, so any such prefix is at best redundant. Remove it and retest.
- Look at the diagnostic fieldThis cause frequently carries the unrecognised network identity in the diagnostic. That tells you exactly which transit network was requested, which is usually enough to identify the rule that inserted it.
- Ask your carrier which transit routes they publishIf the request is coming from your own equipment deliberately, confirm with the carrier whether that route still exists on their network. Often the answer is that it was retired and no replacement identifier is needed.
- Check the gateway is not passing through legacy parametersOn any ISDN to SIP gateway, confirm it is not copying transit selection information into the outbound leg. Most gateways have a setting to drop it, and dropping it is the right default for a UK business trunk.
Fixing the underlying problem
This page explains the code. These guides walk through the fix in detail.
Questions about Q.850 cause 2
How is cause 2 different from cause 3?
Should I ever see cause 2 on a SIP trunk?
Related Q.850 cause values
- Q.850 cause 1Unallocated (unassigned) number
- Q.850 cause 3No route to destination
- Q.850 cause 38Network out of order
Source. Q.850 cause 2 No route to specified transit network (national use) is defined in ITU-T Q.850 section 6.2.7.1.2 and registered in the ITU-T Recommendation Q.850 (10/2018). The causes and fixes above are drawn from our own experience supporting UK business phone systems.