Q.850 cause 1 (Unallocated (unassigned) number): what it means

Short answer: Q.850 cause 1 (Unallocated (unassigned) number) means the network that cleared the call has no subscriber assigned to that number. It is not congestion or a trunk failure: the number does not exist there. It usually comes from a deactivated number or a wrong dialing format. Check the number exactly as it went out in the INVITE.

How Monitor treats cause 1

When the carrier gives the Q.850 cause in the Reason header of the SIP response, Monitor keeps it beside the SIP code on every test call, and it appears in the incident's evidence, ready for the ticket with the carrier. On the English screens, cause 1 reads: the number is not allocated.

Like any failure, a test call read as Down is repeated 3 times, every 2 minutes, and only 4 failures in a row open an incident.

What does cause 1 mean in practice?

Q.850 cause 1, Unallocated (unassigned) number, reports that the destination number reached a switch that does not have it assigned. The format may be correct and the route to that switch may exist; what is missing is an active subscriber with that number. The cause belongs to the normal event class, so it does not indicate an equipment fault. On E1 trunks with SS7, it arrives in the cause parameter of the ISUP REL message; on an ISDN E1 (PRI), in the Q.931 DISCONNECT or RELEASE. In SIP, RFC 3398 (section 7.2.4.1) maps cause 1 to SIP 404 Not Found, and many carriers repeat the original value in the Reason: Q.850;cause=1 header (RFC 6432). In the other direction, 404, 485 and 604 map back to cause 1. In Asterisk it shows up in ${HANGUPCAUSE} and, where the CDR records the field, as hangupcause 1.

When it appears

• A deactivated or canceled number, including an 0800 (toll-free) number taken down when its contract ended.
• A mobile number dialed without the ninth digit, or a landline dialed with one digit too many or too few.
• A PBX (Asterisk, Issabel, FreePBX, 3CX) that normalizes the number incorrectly: it strips the area code, keeps the outbound 0, or sends the carrier selection code (CSP) on a trunk that expects E.164 format.
• A newly contracted or ported DID range that has not yet been activated on the carrier’s switch.
• A ported number whose routing database still points to the old carrier.

How to diagnose it

1. Call the same number from a mobile phone. If that does not complete either, the problem is the number, not your trunk.
2. Capture the signaling with pjsip set logger on (or sip set debug on in chan_sip) and copy the Request-URI and the To header exactly as they went out.
3. Compare the format sent with what the carrier’s contract asks for: +5511..., 11... or 0xx11... with CSP.
4. In sngrep, open the call and look at the final response: a 404 with Reason: Q.850;cause=1 confirms the rejection came from the network.
5. In 3CX, turn on Verbose logging and check in the Activity Log which number went out on the trunk after the dialing rules.
6. If the number is yours (a DID), ask the carrier to confirm that the range is active and routed to your trunk.

What to send the carrier

• Date and time of each attempt, with the time zone (for example, 2026-10-07 14:32, UTC−3).
• The A number (caller) and B number (called) in the format they went out in the INVITE.
• The Call-ID and the full response line, such as SIP/2.0 404 Not Found, with the Reason header.
• If the B number is one of your DID ranges, the contract number or the circuit ID.
• The pcap of the call, if you have it.

Frequently asked questions

Does cause 1 always mean the number does not exist?

It means the network that cleared the call does not recognize the number as assigned. In practice it can be a deactivated number, but also a valid number sent in the wrong format (missing the ninth digit Brazilian mobile numbers carry, missing the area code, with an extra prefix) or a DID range the carrier has not yet activated on its switch.

What is the difference between cause 1 and cause 3?

Cause 1 says the number is not assigned. Cause 3 (No route to destination) says the network has no route to the destination, even though the number may exist on another network. Both become SIP 404 Not Found under RFC 3398, so only the Reason header or the trace on the TDM side shows which of the two happened.