Q.850 cause 6 Channel unacceptable

• Normal class
• Sent by The switch or PBX at either end of a TDM interface
• Rare

What Q.850 cause 6 Channel unacceptable actually means

On a circuit-switched interface, call setup includes agreeing which physical timeslot will carry the audio. This cause is one end saying the timeslot just named cannot be used for this call. It is a negotiation failure between two pieces of equipment rather than anything to do with the caller, the number or the far end. RFC 3398 does not publish a SIP equivalent, since the concept does not exist in SIP; the RFC does say that unmapped causes should default to 500 Server Internal Error, but that is a general fallback rather than a mapping specific to this cause.

This cause indicates that the channel most recently identified is not acceptable to the sending entity for use in this call.

— ITU-T Q.850 section 6.2.7.1.6

The SIP equivalent of Q.850 cause 6

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 6 on a business phone system

• A channel map mismatch across the interfaceOne end configured for a full 30-channel European PRI and the other expecting a different arrangement will disagree about which timeslots exist. Setup proceeds until a channel is named that the other end does not accept.
• Glare between the two endsIf both ends pick channels from the same end of the range, they eventually pick the same one simultaneously. One side then rejects the other choice. Convention is for one end to select from the highest channel down and the other from the lowest up.
• A timeslot out of service or blockedIndividual channels can be busied out for maintenance or blocked automatically after errors. A call offered on a blocked timeslot is refused even though the interface as a whole is working normally.
• A faulty ISDN card or line cardWhere the same small set of channel numbers is always involved, suspect hardware. A card with a degraded timeslot produces intermittent rejections that look random until you correlate them against channel number.

How to fix Q.850 cause 6

1. Compare channel configuration at both endsPut the two configurations side by side and check channel count, numbering and whether the interface is set as network or user side. A mismatch here explains most occurrences and is quick to confirm.
2. Set opposing channel selection to avoid glareConfigure one end to allocate channels ascending and the other descending. If failures cluster at busy times when many channels are in use simultaneously, glare is the likely mechanism and this resolves it.
3. Identify whether specific channels are always involvedRecord the channel number from each failure. A pattern pointing at the same few timeslots means hardware or a stuck block; a spread across all of them means configuration.
4. Question why you still have an ISDN interface at allWith the PSTN switch-off underway, this cause is a signal that a legacy circuit or gateway is still in the path. Plan its removal rather than tuning it, since the underlying service has an end date.

Questions about Q.850 cause 6

Related Q.850 cause values

• Q.850 cause 34No circuit/channel available
• Q.850 cause 44Requested circuit/channel not available
• Q.850 cause 88Incompatible destination

Source. Q.850 cause 6 Channel unacceptable is defined in ITU-T Q.850 section 6.2.7.1.6 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