# ITU-T Q.850 call clearing cause values

Canonical: https://warmtransfer.net/knowledge/q850-cause-codes

Last verified: 2026-09-15

ITU-T Q.850 defines 70 cause values, where every defined value appears as both a row in Table 1 and a numbered subclause under clause 6.2.7[^18]. The in-force components of the standard consist of the 10/2018 edition and Amendment 1 of 04/2019, with the earlier 1993 and 1998 editions categorized as superseded[^14].

## Cause element structure and arithmetic

In the field, a Q.850 cause value consists of a class represented in bits 5 through 7 and an intra-class value in bits 1 through 4, making the resulting cause number equal to the class multiplied by 16 plus that value across all 70 entries in Table 1[^10]. Clause 6.2.5 labels both class 000 and class 001 as "normal event," and clause 6.2.7 consolidates both under a single "Normal class" heading, meaning the 8 class codes use 7 names[^20]. In the field, this normal class contains 25 of the 70 defined causes—the highest count of any class—whereas the interworking class contains 1 cause[^17].

In the field, each class includes an unspecified cause assigned when no other entry in that class applies: 31 normal, 47 resource unavailable, 63 service or option not available, 79 service or option not implemented, 95 invalid message, 111 protocol error, and 127 interworking[^13]. Specifically, Cause 127 interworking unspecified indicates an interworking scenario with a network that fails to supply causes for its actions, preventing ascertainment of the precise cause[^4].

- **Coding standard**: 2 bits identify the coding standard as ITU-T standardized, ISO/IEC, national, or specific to the identified location, and Q.850 specifies that alternate standards should only be utilized if the desired cause value cannot be represented using ITU-T coding[^11].
- **Location**: A distinct 4-bit field contains 8 defined values ranging from user through international network and network beyond interworking point; citing a cause value without this field omits the identity of who cleared the call[^16].
- **Diagnostics**: Diagnostic data accompanying a cause is listed per cause value in Table 1, and Q.850 indicates that diagnostics are not available for every cause[^12].
- **Recommendation**: This subfield is not supported in the ISDN user part, where its default interpretation is Q.763; whenever this field defaults, its octets are omitted rather than populated with zeros[^19].

Causes transported in the ISDN user part that lack national-use markings establish the minimum set required over the international interface[^15].

## Specific cause distinctions

- **Cause 13 (call completed elsewhere)**: This is the single Q.850 cause specifying SIP in its application column rather than DSS 1 or ISUP, with a definition providing the rule for an S-CSCF adding a Reason header field to a CANCEL request when a forked INVITE receives a 2xx response[^5].
- **Cause 16 (normal call clearing)**: Indicates that one of the participating users asked to clear the call, and Q.850 states that under normal conditions the source of this cause is not the network[^6]. WarmTransfer's reading of the sources is that observing a cause 16 paired with a network element in the location field is a notable trace finding, since normal clearing does not typically originate from the network and the record provides no explanation for why the network cleared the call[^7].
- **Cause 18 vs. Cause 19**: Cause 18 (no user responding) applies when a called party fails to respond to call establishment within the allotted time limit, whereas cause 19 (no answer from user) indicates the called party was alerted but did not answer, isolating a non-signaling device from one that alerted[^8].
- **Cause 38 vs. Cause 41**: Both cause 38 (network out of order) and cause 41 (temporary failure) signify incorrect network functioning, differentiated by expected duration: cause 41 invites an immediate retry, whereas cause 38 indicates an immediate retry is unlikely to succeed[^9].

## See also

- See also [CUCM CDR and CMR field reference](https://warmtransfer.net/knowledge/cucm-cdr-field-reference).
- See also [SIP response codes and failure interpretation](https://warmtransfer.net/knowledge/sip-response-codes).

## Applicability

Applies to: ITU-T Recommendation Q.850. Sources checked 2026-09-15.
The in-force specifications comprise the ITU-T Recommendation Q.850 edition of 10/2018 and Amendment 1 of 04/2019, while the 1993 and 1998 editions are superseded[^14].

## What remains uncertain

- Whether a call record carries the coding standard bits at all is not covered by the sources below.
- Whether the SIP mapping derivable from cause codes matches RFC 3398 is not covered by the sources below.
- What Amendment 1 of 04/2019 changes in the cause table is not covered by the sources below.
- Which causes the superseded 1998 edition did not carry is not covered by the sources below.
- How a cause diagnostic field is decoded is not covered by the sources below.

## Sources

[^1]: Unified CM publishes call termination cause codes beyond the Q.850 set, marked Cisco specific in the table, including 125 out of bandwidth, 126 call split, 131 Call Control Discovery PSTN failover and a run of IME failure codes through 148. Source: [Call Detail Records Administration Guide for Cisco Unified Communications Manager Release 12.5(1), complete book, Internet Archive capture of 2025-03-23](https://web.archive.org/web/20250323135843id_/https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/service/12_5_1/cdrdef/cucm_b_cdr-admin-guide-1251.pdf), Call Detail Records chapter, Call Termination Cause Codes, Table 12. Checked 2026-09-15.
[^2]: For on-net call legs Unified CM determines the cause code value and for off-net legs the far-end switch determines it, so a cause on an off-net leg is a foreign network's statement carried through rather than Unified CM's own diagnosis. Source: [Call Detail Records Administration Guide for Cisco Unified Communications Manager Release 12.5(1), complete book, Internet Archive capture of 2025-03-23](https://web.archive.org/web/20250323135843id_/https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/service/12_5_1/cdrdef/cucm_b_cdr-admin-guide-1251.pdf), Call Detail Records chapter, Call Clearing Causes. Checked 2026-09-15.
[^3]: Every one of the 42 SIP rows of the Cisco-specific cause code table has a Q.850 cause value in the low 16 bits of its code, and the guide states the construction for one row by writing the loop-detected code as 0x42000000 plus EXCHANGE_ROUTING_ERROR (field report). Source: [Call Detail Records Administration Guide for Cisco Unified Communications Manager Release 12.5(1), complete book, Internet Archive capture of 2025-03-23](https://web.archive.org/web/20250323135843id_/https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/service/12_5_1/cdrdef/cucm_b_cdr-admin-guide-1251.pdf), Call Detail Records chapter, Table 13 Cisco-Specific Call Termination Cause Codes. Checked 2026-09-15.
[^4]: Cause 127 interworking unspecified indicates interworking with a network that does not provide causes for its actions, so the precise cause cannot be ascertained. Source: [ITU-T Recommendation Q.850 (10/2018), Usage of cause and location in the Digital Subscriber Signalling System No. 1 and the Signalling System No. 7 ISDN user part](https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-Q.850-201810-I!!PDF-E&type=items), clause 6.2.7.7.1 Cause No. 127. Checked 2026-09-15.
[^5]: Cause 13 call completed elsewhere is the one Q.850 cause whose application column reads SIP rather than DSS 1 or ISUP, and its definition is a rule about what an S-CSCF puts in the Reason header field of a CANCEL after a forked INVITE receives a 2xx. Source: [ITU-T Recommendation Q.850 (10/2018), Usage of cause and location in the Digital Subscriber Signalling System No. 1 and the Signalling System No. 7 ISDN user part](https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-Q.850-201810-I!!PDF-E&type=items), Table 1 row for cause 13 and clause 6.2.7.1.10 Cause No. 13. Checked 2026-09-15.
[^6]: Cause 16 normal call clearing indicates that one of the users involved requested that the call be cleared, and Q.850 states that under normal situations the source of this cause is not the network. Source: [ITU-T Recommendation Q.850 (10/2018), Usage of cause and location in the Digital Subscriber Signalling System No. 1 and the Signalling System No. 7 ISDN user part](https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-Q.850-201810-I!!PDF-E&type=items), clause 6.2.7.1.11 Cause No. 16. Checked 2026-09-15.
[^7]: A cause 16 whose location field names a network element is worth stopping on in a trace, because Q.850 says the source of a normal call clearing is not normally the network, and nothing in the record says why the network cleared the call (inferred). Source: [ITU-T Recommendation Q.850 (10/2018), Usage of cause and location in the Digital Subscriber Signalling System No. 1 and the Signalling System No. 7 ISDN user part](https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-Q.850-201810-I!!PDF-E&type=items), clause 6.2.7.1.11 Cause No. 16 read with clause 6.2.3 Location. Checked 2026-09-15.
[^8]: Cause 18 no user responding is used when the called party does not respond to call establishment within the allotted time, while cause 19 no answer from user is used when the called party was alerted and did not answer, so the pair separates a device that never signalled from a device that rang. Source: [ITU-T Recommendation Q.850 (10/2018), Usage of cause and location in the Digital Subscriber Signalling System No. 1 and the Signalling System No. 7 ISDN user part](https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-Q.850-201810-I!!PDF-E&type=items), clauses 6.2.7.1.13 and 6.2.7.1.14. Checked 2026-09-15.
[^9]: Cause 41 temporary failure and cause 38 network out of order both indicate that the network is not functioning correctly and are separated only by expected duration: a 41 invites an immediate retry and a 38 says an immediate retry is not likely to succeed. Source: [ITU-T Recommendation Q.850 (10/2018), Usage of cause and location in the Digital Subscriber Signalling System No. 1 and the Signalling System No. 7 ISDN user part](https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-Q.850-201810-I!!PDF-E&type=items), clauses 6.2.7.2.2 and 6.2.7.2.5. Checked 2026-09-15.
[^10]: A Q.850 cause value is a class in bits 5 to 7 and a value within the class in bits 1 to 4, so the cause number is the class times 16 plus the value, and that arithmetic holds for all 70 cause values in Table 1 (field report). Source: [ITU-T Recommendation Q.850 (10/2018), Usage of cause and location in the Digital Subscriber Signalling System No. 1 and the Signalling System No. 7 ISDN user part](https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-Q.850-201810-I!!PDF-E&type=items), clause 6.2.5 Cause value and Table 1. Checked 2026-09-15.
[^11]: Two bits of the cause element select the coding standard as ITU-T standardized, ISO/IEC, national, or specific to the identified location, and Q.850 states the other standards should be used only when the desired cause value cannot be represented with ITU-T coding. Source: [ITU-T Recommendation Q.850 (10/2018), Usage of cause and location in the Digital Subscriber Signalling System No. 1 and the Signalling System No. 7 ISDN user part](https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-Q.850-201810-I!!PDF-E&type=items), clause 6.2.2 Coding standard. Checked 2026-09-15.
[^12]: The diagnostics that may accompany a cause are given per cause value in Table 1 and Q.850 states that diagnostic information is not available for every cause. Source: [ITU-T Recommendation Q.850 (10/2018), Usage of cause and location in the Digital Subscriber Signalling System No. 1 and the Signalling System No. 7 ISDN user part](https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-Q.850-201810-I!!PDF-E&type=items), clause 6.2.6 Diagnostics. Checked 2026-09-15.
[^13]: Each Q.850 class carries an unspecified cause used only when no other cause in that class applies, and the definitions say so in the same words for each: 31 normal, 47 resource unavailable, 63 service or option not available, 79 service or option not implemented, 95 invalid message, 111 protocol error and 127 interworking (field report). Source: [ITU-T Recommendation Q.850 (10/2018), Usage of cause and location in the Digital Subscriber Signalling System No. 1 and the Signalling System No. 7 ISDN user part](https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-Q.850-201810-I!!PDF-E&type=items), clauses 6.2.7.1.25, 6.2.7.2.10, 6.2.7.3.8, 6.2.7.4.5, 6.2.7.6.10 and 6.2.7.7.1. Checked 2026-09-15.
[^14]: The in-force components of Q.850 are the edition of 10/2018 and Amendment 1 of 04/2019, and the 1993 and 1998 editions are listed as superseded. Source: [ITU-T Recommendation Q.850 (10/2018), Usage of cause and location in the Digital Subscriber Signalling System No. 1 and the Signalling System No. 7 ISDN user part](https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-Q.850-201810-I!!PDF-E&type=items), ITU-T recommendation landing page, In force components table. Checked 2026-09-15.
[^15]: Causes carried in the ISDN user part and not marked for national use are the minimum set of cause values that shall be supported over the international interface. Source: [ITU-T Recommendation Q.850 (10/2018), Usage of cause and location in the Digital Subscriber Signalling System No. 1 and the Signalling System No. 7 ISDN user part](https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-Q.850-201810-I!!PDF-E&type=items), Table 1 note 1. Checked 2026-09-15.
[^16]: Location is a separate four-bit field of the same cause element with 8 defined values from user through international network and network beyond interworking point, so a cause value quoted without its location does not say who cleared the call. Source: [ITU-T Recommendation Q.850 (10/2018), Usage of cause and location in the Digital Subscriber Signalling System No. 1 and the Signalling System No. 7 ISDN user part](https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-Q.850-201810-I!!PDF-E&type=items), clause 6.2.3 Location. Checked 2026-09-15.
[^17]: The normal class holds 25 of the 70 Q.850 causes, more than any other class, and the interworking class holds exactly one (field report). Source: [ITU-T Recommendation Q.850 (10/2018), Usage of cause and location in the Digital Subscriber Signalling System No. 1 and the Signalling System No. 7 ISDN user part](https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-Q.850-201810-I!!PDF-E&type=items), clause 6.2.7 subclause groupings. Checked 2026-09-15.
[^18]: ITU-T Q.850 defines 70 cause values and every one of them is both a row of Table 1 and a numbered definition subclause under clause 6.2.7; the two lists are identical. Source: [ITU-T Recommendation Q.850 (10/2018), Usage of cause and location in the Digital Subscriber Signalling System No. 1 and the Signalling System No. 7 ISDN user part](https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-Q.850-201810-I!!PDF-E&type=items), Table 1 Cause information element/parameter and clause 6.2.7 Cause definitions. Checked 2026-09-15.
[^19]: The Recommendation subfield of the cause element is not supported by the ISDN user part and its default interpretation there is Q.763, and when the field defaults the octets carrying it are omitted entirely rather than sent as zero. Source: [ITU-T Recommendation Q.850 (10/2018), Usage of cause and location in the Digital Subscriber Signalling System No. 1 and the Signalling System No. 7 ISDN user part](https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-Q.850-201810-I!!PDF-E&type=items), Figure 1 Format of Cause, notes 1 and 2, and clause 6.2.4. Checked 2026-09-15.
[^20]: Clause 6.2.5 names class 000 and class 001 both as normal event, and clause 6.2.7 groups both under a single heading called the Normal class, so the eight class codes carry seven names. Source: [ITU-T Recommendation Q.850 (10/2018), Usage of cause and location in the Digital Subscriber Signalling System No. 1 and the Signalling System No. 7 ISDN user part](https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-Q.850-201810-I!!PDF-E&type=items), clause 6.2.5 Cause value, class list, and clause 6.2.7.1 Normal class. Checked 2026-09-15.