Q.850 cause 1 Unallocated (unassigned) number

• Normal class
• Sent by The terminating carrier, or the switch holding that number range
• Common

What Q.850 cause 1 Unallocated (unassigned) number actually means

The dialled digits passed the format check — the right length, a recognised prefix, a range the network knows about — but when the terminating switch looked the number up, no subscriber was attached to it. That distinction matters when you are reading a CDR. Cause 28 says the digits themselves were wrong; cause 1 says the digits were fine and the number simply is not in service. Under RFC 3398 a gateway converts this to SIP 404 Not Found.

This cause indicates that the called party cannot be reached because, although the called party number is in a valid format, it is not currently allocated (assigned).

— ITU-T Q.850 section 6.2.7.1.1

The SIP equivalent of Q.850 cause 1

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 1 on a business phone system

• A transposed or missing digitBy far the most common reason. UK geographic numbers are long enough that a single wrong digit still produces a valid-looking number, so the network accepts it and then finds nobody there. Check the exact digit string in the log rather than the number the caller believes they dialled.
• The line has been ceasedWhen a business closes an account, the number goes back to the range holder and stays unallocated until it is reissued. Old marketing material, invoices and Google listings keep sending callers to it for years afterwards.
• A DID that was never provisionedYou bought a block of numbers but only some were actually built on the trunk. The range holder answers for the whole block, so calls to the unbuilt ones return unallocated rather than failing further upstream.
• A port that did not complete cleanlyDuring a number port the losing and gaining carriers both have to act. If the import step stalls, the number can end up owned by nobody for a period, and calls to it come back as unallocated.
• An internal extension leaking to the PSTNA three or four digit internal number that does not match any dial plan rule can get prefixed with an outbound route and pushed to the carrier. It arrives as a nonsense national number and is refused.

How to fix Q.850 cause 1

1. Read the digits the system actually sentTake them from the CDR or the SIP trace, not from the person who made the call. If those digits are wrong, the problem is upstream of the carrier and no amount of carrier investigation will help.
2. Dial the same number from a mobileA mobile uses a completely different route and dial plan. If it also fails, the number is genuinely not in service. If it connects, the fault is in your own routing or your carrier.
3. Confirm the DID is built on your trunkFor inbound failures, check the number appears in your carrier portal as routed to your trunk, and that your PBX has an inbound rule matching it in the format the carrier presents.
4. Check the dial plan for stray prefixesLook for rules that prepend a route access code or strip a leading zero twice. Place a test call and compare what the user pressed with what left the system.
5. Ask the carrier to query the range holderIf the digits are right and a mobile also fails, your carrier can confirm which operator holds the range and whether the number is allocated. That closes the question definitively.

Questions about Q.850 cause 1

Related Q.850 cause values

• Q.850 cause 22Number changed
• Q.850 cause 28Invalid number format (address incomplete)
• Q.850 cause 3No route to destination

Source. Q.850 cause 1 Unallocated (unassigned) number is defined in ITU-T Q.850 section 6.2.7.1.1 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.

All Call hangup causes (Q.850) codes