Q.850 cause 44 Requested circuit/channel not available

What Q.850 cause 44 means

The far end rejected the particular circuit your equipment chose, not the call itself. RFC 3398 treats cause 44 as untranslatable: the correct behaviour is for the originating switch to pick a different circuit and try again, so no SIP status should be sent at all. In a call record it points at a TDM gateway rather than at anything your users touch.

Resource unavailable class
Sent by The far side of a TDM interface, usually a gateway or an exchange
Rare

What Q.850 cause 44 Requested circuit/channel not available actually means

Two pieces of equipment sharing a digital circuit each keep a view of every timeslot on it, and this cause means those views disagree. The originating side chose a circuit identification code and the receiving side believed that circuit to be in an incompatible state, so it refused it. RFC 3398 discusses cause 44 explicitly as one that should not be translated into a SIP response, precisely because tearing the call down is the wrong answer; retrying on another circuit is the right one. If you do see a SIP status attached to it, that is an implementation flattening the cause rather than following the recommendation.

This cause is returned when the circuit or channel indicated by the requesting entity cannot be provided by the other side of the interface.
— ITU-T Q.850 section 6.2.7.2.8

The SIP equivalent of Q.850 cause 44

RFC 3398 publishes no recommended SIP status for this cause. It maps a subset of cause values only, and states that a cause outside that subset should default to 500 Server Internal Error. Seeing this cause reach a SIP leg unchanged usually means something in the path passed it through without translating it.

What causes Q.850 cause 44 on a business phone system

Circuit state mismatch after a fault or rebootOne end brings timeslots back into service believing them idle while the other still has them blocked. Calls fail on the disputed timeslots and succeed on the rest, which makes the failure look intermittent when it is actually specific.
Glare, where both ends seize the same timeslot at onceOn a two-way circuit an inbound and an outbound call can grab the same channel in the same instant. Equipment is meant to resolve this by convention, one end selecting from the top of the range and the other from the bottom, and glare gets frequent when both are configured the same way.
A channel map that does not match the carrierIf your gateway is configured for a different set of channels than the exchange has provisioned, calls on the mismatched ones fail while others work. This tends to show up immediately after an install or a capacity change.
A partially recovered E1 or ISDN30 circuitAfter a line fault, some timeslots can return to service while others stay blocked at one end. The circuit tests as up and carries calls, which is exactly why the residual problem goes unnoticed.

How to fix Q.850 cause 44

Check whether the call completes on a second attemptIf it does, the equipment is doing what the specification intends and picking a different circuit. That makes the issue a housekeeping matter rather than a service-affecting fault, though it is still worth clearing.
Compare circuit state on both endsLook at which timeslots each side considers blocked, idle or in use, and identify the ones they disagree about. The disagreement itself is the fault, and it is usually confined to a handful of channels.
Reset the disputed timeslots in a maintenance windowA circuit group reset resynchronises both views, but it will clear calls on those channels. Do it outside business hours and confirm afterwards that both ends agree.
Check the channel selection convention at each endWhere glare is the pattern, having one end hunt upwards and the other downwards removes most collisions. Get this agreed with the carrier rather than changing it unilaterally.
Ask whether the TDM leg still needs to existThis cause only occurs where circuit-switched interfaces remain in the path. With ISDN services being withdrawn, removing that leg solves this and a family of related problems permanently.

Fixing the underlying problem

This page explains the code. These guides walk through the fix in detail.

Questions about Q.850 cause 44

Why is there no SIP code for cause 44?

Because RFC 3398 deliberately leaves it untranslated. The cause is a request to try a different circuit, which has no meaning in SIP, and converting it into a failure response would tell the caller the call cannot be completed when in fact it usually can.

Does cause 44 mean all my lines are busy?

No, that is a different cause. Cause 34, No circuit/channel available, means nothing was free; cause 44 means a specific circuit was named and the far end would not accept that one.

Will users notice cause 44?

Often not, because well-behaved equipment retries on another circuit and the call connects normally. It usually turns up when someone is reading call records or chasing an unrelated fault.

Related Q.850 cause values

Source. Q.850 cause 44 Requested circuit/channel not available is defined in ITU-T Q.850 section 6.2.7.2.8 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