Q.850 cause 55 Incoming calls barred within CUG

What Q.850 cause 55 means

A Closed User Group is an ISDN-era private calling community, and cause 55 says the called line does belong to the right group but is not allowed to receive incoming calls within it. It maps to SIP 403 Forbidden. On a modern hosted phone system this is genuinely rare and usually means old ISDN configuration is still in the path.

Service or option unavailable class
Sent by The terminating network, on behalf of the called CUG member
Rare

What Q.850 cause 55 Incoming calls barred within CUG actually means

A Closed User Group, or CUG, is a set of lines that a network treats as a private community, with separate permissions controlling whether each member may originate calls into the group, receive them, or reach the outside world. Businesses used them for private multi-site dialling before VPNs and hosted platforms made the idea unnecessary. This cause is specifically about a member in good standing whose incoming permission within the group is switched off, and RFC 3398 maps it to 403 Forbidden. Be aware that 403 is a broad response in SIP, so the Q.850 value is what identifies this as a CUG matter at all.

This cause indicates that although the called party is a member of the CUG for the incoming CUG call, incoming calls are not allowed to this member of the CUG.
— ITU-T Q.850 section 6.2.7.3.4

The SIP equivalent of Q.850 cause 55

A gateway translating this cause into SIP reports it as SIP 403 Forbidden. The mapping is defined in RFC 3398 section 7.2.4.1.

What causes Q.850 cause 55 on a business phone system

A surviving CUG on an ISDN serviceGroups configured years ago remain in carrier records long after anyone remembers ordering them. They only become visible when the calling pattern changes and someone dials a member that has never received an incoming group call.
Outgoing-only permissions on the destinationA line set up to originate calls within the group but not accept them behaves exactly as configured. Whoever set it that way may have intended it, for example on a line dedicated to a device that only ever dials out.
Group data carried across during a port or migrationNumber ports sometimes bring legacy service attributes with them. The new service inherits restrictions nobody asked for, and they surface as odd, selective call failures after the cutover.
A carrier private numbering plan behaving like a CUGSome wholesale platforms implement private dialling arrangements using the same underlying mechanism. The label on the order form may say something else entirely while the signalling still reports a CUG restriction.

How to fix Q.850 cause 55

Confirm whether a real CUG exists on the serviceAsk the carrier directly whether the number is in a Closed User Group and what its permissions are. This takes one question and saves a great deal of speculative troubleshooting on equipment that is behaving correctly.
Check the incoming permission on the called memberThe restriction lives on the destination, not on the caller, so investigating the calling system will find nothing. Have the permission changed if the line is supposed to accept group calls.
Ask whether the group is still needed at allSite-to-site dialling on a hosted platform, and short-code dialling between users, deliver what CUGs were invented for without the restrictions. Retiring the group often removes the whole class of problem.
Remove legacy group data as part of any migrationPut it explicitly on the cutover checklist rather than assuming a new service starts clean. Legacy attributes that survive a migration are hard to spot afterwards because nobody expects them to be there.

Fixing the underlying problem

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

Questions about Q.850 cause 55

What is a Closed User Group?

It is a network-level private calling community from the ISDN era, where a defined set of lines can call each other under permissions the carrier holds. Each member has separate rights to make and receive calls within the group, which is why two different causes exist for group refusals.

What is the difference between cause 55 and cause 87?

Cause 55 accepts that the called party is a member and refuses because that member cannot take incoming group calls. Cause 87 is about membership itself, meaning the called party is not in the specified group, or an ordinary subscriber has dialled a group subscriber.

Should a hosted VoIP customer ever see cause 55?

Rarely, and when it happens something ISDN-derived is usually still involved somewhere in the route. It is worth checking rather than assuming, but it is not a code to design your troubleshooting around.

Related Q.850 cause values

Source. Q.850 cause 55 Incoming calls barred within CUG is defined in ITU-T Q.850 section 6.2.7.3.4 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