Q.850 cause 70 Only restricted digital information bearer capability is available

What Q.850 cause 70 means

The call asked for a clear 64 kbit/s digital path and the equipment can only offer the restricted version, where part of the capacity is reserved for signalling. Cause 70 belongs to old data calls rather than telephony, and it maps to SIP 488 Not Acceptable Here. A UK business meets it, if at all, on international or legacy data routes.

Service or option not implemented class
Sent by A far-end or transit switch describing what it can offer
Rare

What Q.850 cause 70 Only restricted digital information bearer capability is available actually means

Unrestricted digital information means every bit of the 64 kbit/s channel carries user data. Restricted means the network needs to borrow from the data stream for its own signalling, historically by taking a bit from each sample on older North American carrier systems, leaving about 56 kbit/s usable. This cause is the network saying it can carry the call but only in the restricted form, and RFC 3398 maps it to 488 Not Acceptable Here. It is a genuinely specialised code: it has no bearing on ordinary speech calls, and seeing it means real digital bearer traffic is in play.

This cause indicates that the calling party has requested an unrestricted bearer service but that the equipment sending this cause only supports the restricted version of the requested bearer capability.
— ITU-T Q.850 section 6.2.7.4.4

The SIP equivalent of Q.850 cause 70

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 70 on a business phone system

An international route that cannot guarantee bit transparencyOlder transit arrangements, particularly across some transatlantic paths, only ever offered the restricted service. The limitation belongs to a network in the middle rather than to either end.
Legacy ISDN data equipment still in serviceVideoconferencing codecs, backup dial links and machine-to-machine data units set up in the ISDN era request unrestricted bearers by default. Very little of this equipment remains, but what does remain tends to be in industrial or broadcast settings.
A PBX signalling a digital bearer for speech callsMisconfiguration can put a voice call into the digital bearer category where it does not belong. The remedy is to correct the signalling rather than to chase the restriction.
A gateway that only implements the restricted variantSome interworking equipment supports restricted digital operation and not the unrestricted form. It is an honest statement of a limit rather than a fault to be cleared.

How to fix Q.850 cause 70

Confirm whether the call is really a data callIf a person was dialling to speak to someone, the bearer request is wrong and that is the fault. Correcting it removes the cause entirely rather than working around it.
Decide whether the application tolerates restricted operationSome equipment can be configured to accept the restricted rate at reduced throughput. Where the link is a low-volume backup path, that is often perfectly acceptable.
Ask the carrier for a route that supports the unrestricted bearerWhere the limitation lies with a transit network, a different route may avoid it. Provide the exact destination numbers, because the answer varies by country and by wholesale supplier.
Plan the retirement of the ISDN data pathWith ISDN services being withdrawn ahead of the switch-off, this is a matter of when rather than whether. An IP replacement for the same function will be cheaper and will not raise bearer questions at all.

Fixing the underlying problem

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

Questions about Q.850 cause 70

What is the difference between restricted and unrestricted digital information?

Unrestricted gives the application the full 64 kbit/s with every bit preserved. Restricted reserves part of that capacity for the network's own signalling, leaving roughly 56 kbit/s, which matters for data equipment and not at all for speech.

Will cause 70 affect normal phone calls?

It should not, because speech calls do not request an unrestricted digital bearer. If it appears on ordinary calls, something in your signalling is describing them as data and that misdescription is the real fault.

Related Q.850 cause values

Source. Q.850 cause 70 Only restricted digital information bearer capability is available is defined in ITU-T Q.850 section 6.2.7.4.4 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