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Q.850 cause 65 Bearer capability not implemented

What Q.850 cause 65 means

The far end does not support the type of connection the call asked for at all. This is a capability limit rather than a permissions refusal, and it maps to SIP 488 Not Acceptable Here, the same response you see when two devices cannot agree a codec. On a UK hosted system it turns up mostly on fax, modem and legacy data calls meeting an ISDN gateway.

What Q.850 cause 65 Bearer capability not implemented actually means

The equipment is stating a hard limit about itself: it has no implementation of the bearer capability requested, so the request can never succeed against that device however it is provisioned. RFC 3398 maps it to 488 Not Acceptable Here, which is worth knowing because 488 is also what SIP devices send when there is no codec in common. When you find a 488 carrying a Reason header with Q.850 cause 65, you are looking at a media or bearer negotiation failure rather than anything to do with capacity or barring.

This cause indicates that the equipment sending this cause does not support the bearer capability requested.

— ITU-T Q.850 section 6.2.7.4.1

The SIP equivalent of Q.850 cause 65

A gateway translating this cause into SIP reports it as SIP 488 Not Acceptable Here. The mapping is defined in RFC 3398 section 7.2.4.1.

What causes Q.850 cause 65 on a business phone system

How to fix Q.850 cause 65

  1. Establish whether the call is voice, fax or dataThe answer changes everything that follows, and it is often obvious from which extension or device originated the call. Treat fax and modem traffic as a separate problem from telephony.
  2. Compare the codec lists at both endsList what each side offers and confirm there is an overlap. In the UK, having G.711 A-law enabled on both ends as a common fallback resolves the majority of these failures.
  3. Give fax traffic a path designed for itUse T.38 where both ends support it, or move the workflow to email. Fax over a compressed voice codec is unreliable even when the call does connect, so a working call is not the same as a solved problem.
  4. Check the settings on any analogue adaptor involvedAn ATA sitting in front of a fax machine or card terminal controls how the call is presented. Its fax and codec settings are a frequent source of unsupported requests.
  5. Check firmware if the request looks correct but is still refusedSupport for a bearer or codec sometimes arrives in a later release. This is worth checking last rather than first, because upgrades carry their own risk.

Fixing the underlying problem

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

Questions about Q.850 cause 65

Does cause 65 mean a codec problem?
On an IP network, usually yes. Its SIP equivalent of 488 Not Acceptable Here is the standard response when two devices share no common codec, and that is the most likely reading unless legacy data equipment is involved.
Why do fax calls produce cause 65?
Fax needs either a preserved audio path or a proper T.38 relay, and it may request a bearer the voice path never implemented. Rather than making the refusal go away, it is generally better to give fax its own arrangement.
Can I fix cause 65 by changing my subscription?
No, and that is what separates it from cause 57. Cause 65 says the equipment has no such capability, so the change has to be to the configuration, the equipment or the route.

Related Q.850 cause values

Source. Q.850 cause 65 Bearer capability not implemented is defined in ITU-T Q.850 section 6.2.7.4.1 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

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