Q.850 cause 50 Requested facility not subscribed

What Q.850 cause 50 means

The feature the call asked for exists on the equipment, but this line is not signed up to use it. Cause 50 is a provisioning and billing answer rather than a technical fault, and the fix is a change to the account rather than to the phone. Barring on international, premium-rate or mobile destinations is the usual reason in the UK.

Service or option unavailable class
Sent by The network element checking your subscription, normally the originating exchange
Rare

What Q.850 cause 50 Requested facility not subscribed actually means

A supplementary service is anything layered on top of a basic call, such as diverting to an external number, three-way calling or calling a category of destination the account is restricted from. This cause says the equipment implements that service perfectly well but the user is not authorised to use it, which is a deliberate policy answer rather than a failure. RFC 3398 publishes no SIP mapping for cause 50, so do not expect a consistent SIP status; the general rule in the RFC is that unmapped causes default to 500 Server Internal Error, but implementations vary and you should not rely on the SIP side to identify it.

This cause indicates that the user has requested a supplementary service which is implemented by the equipment which generated this cause, but which the user is not authorized to use.
— ITU-T Q.850 section 6.2.7.3.2

The SIP equivalent of Q.850 cause 50

RFC 3398 publishes no recommended SIP status for this cause. It maps a subset of cause values only, and states that a cause outside that subset should default to 500 Server Internal Error. Seeing this cause reach a SIP leg unchanged usually means something in the path passed it through without translating it.

What causes Q.850 cause 50 on a business phone system

Call barring applied to the accountInternational, premium-rate, directory-enquiry and mobile destinations are frequently barred by default on business accounts. The bar is invisible until someone dials a barred category for the first time.
A supplementary service outside the tariffFeatures such as diverting to an external mobile can be chargeable extras rather than standard inclusions. The PBX offers the option, the carrier declines to carry it out.
A feature switched on locally but never ordered upstreamEnabling something on your own system does not provision it with the carrier. This is a common source of confusion after a configuration change, because the setting looks correct at your end.
Anti-fraud barring imposed after an incidentFollowing suspected toll fraud, providers often bar high-risk destination ranges without a great deal of announcement. If barring appeared suddenly and applies to expensive international ranges, ask whether it was applied protectively.
A destination range never opened on the accountSome ranges, particularly non-geographic and international premium services, need explicit enabling. The account is working normally; that one category was simply never turned on.

How to fix Q.850 cause 50

Identify precisely what was being requestedNote the dialled number and whether a feature such as diversion was involved. A barred destination category and a barred service are different conversations with the provider.
Read the barring and feature list on the accountMost provider portals show which categories are enabled per line or per trunk. Compare a line that works with the line that does not, since the difference is usually visible in that comparison alone.
Decide whether the bar should stayBarring is often there for good reason, particularly on international ranges that attract fraud. Removing it because a user complained can be an expensive way to resolve a ticket.
Request the change in writing and get it confirmedAsk the provider to enable the specific service or unbar the specific range, and note who authorised it. Changes to barring affect cost exposure, so a record matters later.
Retest with a known destination in the same categoryAfter the change, dial something in the affected range rather than a general test number. Enabling one category does not enable a neighbouring one.

Fixing the underlying problem

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

Questions about Q.850 cause 50

Is cause 50 a fault or a billing matter?

Almost always billing or provisioning. The equipment is capable and is refusing on purpose because the account is not authorised, so the resolution comes from your account manager rather than from support.

Why did calls that used to work start returning cause 50?

Something changed on the account rather than on the network. The two usual explanations are a tariff or package change and protective barring applied after suspected fraud, and the provider can confirm which within a single call.

Which SIP response corresponds to cause 50?

RFC 3398 does not publish one. Gateways therefore differ in what they send, and the RFC's general fallback for unlisted causes is 500 Server Internal Error, so treat the SIP status as unreliable for identifying this particular cause.

Related Q.850 cause values

Source. Q.850 cause 50 Requested facility not subscribed is defined in ITU-T Q.850 section 6.2.7.3.2 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