Q.850 cause 87 User not member of CUG

• Invalid message class
• Sent by The terminating exchange enforcing the group restriction
• Rare

What Q.850 cause 87 User not member of CUG actually means

Where cause 55 concerns permissions inside a group you belong to, this one is about membership itself, and the definition covers both directions of that failure. It applies when a called party turns out not to be in the group the call specified, and equally when an ordinary subscriber outside any group calls a line that only accepts group traffic. RFC 3398 maps it to 403 Forbidden. Modern platforms achieve the same effect with tenant boundaries and dial-plan restrictions, so a refusal that looks like this on a hosted system is usually a policy rule wearing an old label.

This cause indicates that the called user for the incoming CUG call is not a member of the specified CUG or that the calling user is an ordinary subscriber calling a CUG subscriber.

— ITU-T Q.850 section 6.2.7.5.7

The SIP equivalent of Q.850 cause 87

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 87 on a business phone system

• Dialling a line that only accepts calls from its own groupThe destination is deliberately unreachable from the ordinary telephone network. This is working as intended, and the person to speak to is whoever owns the destination rather than your provider.
• A member removed from the group during a changeGroup membership is carrier-held data that can be lost during a reconfiguration or a billing change. Calls between sites that worked yesterday stop working today with nothing having changed on either PBX.
• The originating system signalling the wrong group indexWhere a PBX inserts a group identifier into outgoing calls, an incorrect value names a group the destination is not in. The destination is refusing accurately; the question it was asked was wrong.
• A modern tenant or dial-plan restriction reported in old termsHosted platforms restrict which extensions and sites can reach each other, and some gateways express that refusal with a CUG cause. Take the code as a hint about the shape of the problem rather than as proof that a real CUG exists.

How to fix Q.850 cause 87

1. Determine whether a real CUG is involvedAsk the carrier whether either number is in a Closed User Group. If neither is, you are looking at a modern access restriction and should be searching your own dial plan instead.
2. Check membership of both numbers, not just the destinationThe definition covers a non-member being called and an outsider calling in. Confirming both sides at once avoids a second round of questions with the provider.
3. Verify what group identifier your system sendsA trace of the outgoing setup shows the group information the call carries. An incorrect or stale value is easy to fix once you can see it.
4. Replace the group with modern access controls when migratingSite-to-site dialling, extension ranges and class-of-service rules on a hosted platform give you the same control and are visible in a portal. Plan the swap rather than porting the old restriction forward.

Questions about Q.850 cause 87

Related Q.850 cause values

• Q.850 cause 50Requested facility not subscribed
• Q.850 cause 55Incoming calls barred within CUG
• Q.850 cause 63Service or option not available, unspecified

Source. Q.850 cause 87 User not member of CUG is defined in ITU-T Q.850 section 6.2.7.5.7 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