Author’s note: This article supports our legacy products. At Inventive Labs, after two decades of providing telephony tools, we never stop supporting those who rely on our products to run their businesses. We no longer recommend Dialogic. This article is for our customers who are using legacy equipment. For more information about these legacy products and where we are today, read Ditch Dialogic & Convert to Voice Elements.
Q.931 was designed for ISDN call establishment, maintenance, and release of network connections between two DTEs on the ISDN D channel. Q.931 has more recently been used as part of the VoIP Call Causes as shown below. In troubleshooting, the code standards below may be helpful.
Message examples
- SETUP (indicating the establishment of a connection)
- CALL PROCEEDING (indicating that the call is being processed by the destination terminal)
- ALERTING (tells the calling party that the destination terminal is ringing)
- CONNECT (sent back to the calling party indicating that the intended destination has answered the call)
- DISCONNECT (sent to indicate a request to terminate the connection, by the end that seeks to terminate)
- RELEASE (sent in response to the disconnect request indicating that the call is to be terminated).
- RELEASE COMPLETE (sent by the receiver of the release to complete the handshake).
Disconnect causes
| Hex | Decimal | Cause |
|---|---|---|
| 0x1 | 1 | Unallocated or unassigned number |
| 0x2 | 2 | No route to specified transit network (Transit Network Identity) |
| 0x3 | 3 | No route to destination |
| 0x4 | 4 | Send special information tone |
| 0x5 | 5 | Misdialed trunk prefix |
| 0x6 | 6 | Channel unacceptable |
| 0x7 | 7 | Call awarded and being delivered in an established channel |
| 0x8 | 8 | Prefix 0 dialed but not allowed |
| 0x9 | 9 | Prefix 1 dialed but not allowed |
| 0xA | 10 | Prefix 1 not dialed but required |
| 0xB | 11 | More digits received than allowed, call is proceeding |
| 0x10 | 16 | Normal call clearing |
| 0x11 | 17 | User busy |
| 0x12 | 18 | No user responding |
| 0x13 | 19 | T.301 expired: – User Alerted, No answer from user |
| 0x15 | 21 | Call rejected |
| 0x16 | 22 | Number changed to number in diagnostic field. |
| 0x17 | 23 | Reverse charging rejected |
| 0x18 | 24 | Call suspended |
| 0x19 | 25 | Call resumed |
| 0x1A | 26 | Non-selected user clearing |
| 0x1B | 27 | Destination out of order |
| 0x1C | 28 | Invalid number format or incomplete address |
| 0x1D | 29 | EKTS facility rejected by network |
| 0x1E | 30 | Response to STATUS INQUIRY |
| 0x1F | 31 | Normal, unspecified |
| 0x21 | 33 | Circuit out of order |
| 0x22 | 34 | No circuit/channel available |
| 0x23 | 35 | Destination unattainable |
| 0x24 | 36 | Out of order |
| 0x25 | 37 | Degraded service |
| 0x26 | 38 | Network out of order |
| 0x27 | 39 | Transit delay range cannot be achieved |
| 0x28 | 40 | Throughput range cannot be achieved |
| 0x29 | 41 | Temporary failure |
| 0x2A | 42 | Switching equipment congestion |
| 0x2B | 43 | Access information discarded |
| 0x2C | 44 | Requested circuit channel not available |
| 0x2D | 45 | Preempted |
| 0x2E | 46 | Precedence call blocked |
| 0x2F | 47 | Resource unavailable, unspecified |
| 0x31 | 49 | Quality of service unavailable |
| 0x32 | 50 | Requested facility not subscribed |
| 0x33 | 51 | Reverse charging not allowed |
| 0x34 | 52 | Outgoing calls barred |
| 0x35 | 53 | Outgoing calls barred within CUG |
| 0x36 | 54 | Incoming calls barred |
| 0x37 | 55 | Incoming calls barred within CUG |
| 0x38 | 56 | Call waiting not subscribed |
| 0x39 | 57 | Bearer capability not authorized |
| 0x3A | 58 | Bearer capability not presently available |
| 0x3F | 63 | Service or option not available, unspecified |
| 0x41 | 65 | Bearer service not implemented |
| 0x42 | 66 | Channel type not implemented |
| 0x43 | 67 | Transit network selection not implemented |
| 0x44 | 68 | Message not implemented |
| 0x45 | 69 | Requested facility not implemented |
| 0x46 | 70 | Only restricted digital information bearer capability is available |
| 0x4F | 79 | Service or option not implemented, unspecified |
| 0x51 | 81 | Invalid call reference value |
| 0x52 | 82 | Identified channel does not exist |
| 0x53 | 83 | A suspended call exists, but this call identity does not |
| 0x54 | 84 | Call identity in use |
| 0x55 | 85 | No call suspended |
| 0x56 | 86 | Call having the requested call identity has been cleared |
| 0x57 | 87 | Called user not member of CUG |
| 0x58 | 88 | Incompatible destination |
| 0x59 | 89 | Non-existent abbreviated address entry |
| 0x5A | 90 | Destination address missing, and direct call not subscribed |
| 0x5B | 91 | Invalid transit network selection (national use) |
| 0x5C | 92 | Invalid facility parameter 93 Mandatory information element is missing |
| 0x5D | 93 | Message type non-existent or not implemented |
| 0x5F | 95 | Invalid message, unspecified |
| 0x60 | 96 | Mandatory information element is missing |
| 0x61 | 97 | Message type non-existent or not implemented |
| 0x62 | 98 | Message not compatible with call state or message type non-existent or not implemented |
| 0x63 | 99 | Information element nonexistent or not implemented |
| 0x64 | 100 | Invalid information element contents |
| 0x65 | 101 | Message not compatible with call state |
| 0x66 | 102 | Recovery on timer expiry |
| 0x67 | 103 | Parameter non-existent or not implemented – passed on |
| 0x6F | 111 | Protocol error, unspecified |
| 0x7F | 127 | Internetworking, unspecified |
| 0x80+ | 128 orhigher | Proprietary diagnostic code (not necessarily bad). Typically used to pass proprietary control or maintenance messages between multiplexers. |