Q.850 cause 20 Subscriber absent

• Normal class
• Sent by A mobile network, or a platform tracking where a user is logged in
• Occasional

What Q.850 cause 20 Subscriber absent actually means

This cause was written for mobility. The specification describes a mobile station that has logged off or cannot be contacted by radio, and extends the same idea to any personal telecommunications user who is temporarily not addressable at any interface. The word temporarily is doing a lot of work: nothing is broken and nothing is barred, the subscriber is simply not present on the network at this moment. RFC 3398 maps it to SIP 480 Temporarily Unavailable, the same status as cause 19, which is why you need the Q.850 value to tell the two situations apart in a CDR.

This cause value is used when a mobile station has logged off, radio contact is not obtained with a mobile station or if a personal telecommunication user is temporarily not addressable at any user-network interface.

— ITU-T Q.850 section 6.2.7.1.15

The SIP equivalent of Q.850 cause 20

A gateway translating this cause into SIP reports it as SIP 480 Temporarily Unavailable. The mapping is defined in RFC 3398 section 7.2.4.1.

What causes Q.850 cause 20 on a business phone system

• The mobile is switched off or in flight modeThe handset has deregistered from the network cleanly, so the network knows there is no point paging it. Calls go straight to the mobile voicemail if one is set, or return this cause if not.
• No radio coverage at the handsetBasements, lifts, rural sites and industrial buildings all produce this. The subscriber record is perfectly healthy; the network simply cannot raise the device. It resolves itself when they walk outside.
• A SIM that has been ceased or suspendedA leaver whose SIM has been cancelled, or an account suspended for non-payment, produces the same unreachable result. Worth checking against your mobile inventory before chasing it as a fault.
• A hot-desk user not logged in anywhereOn systems where an extension follows the person rather than the handset, a user who has not logged into a phone has no device associated with them. The platform reports the user as absent rather than unanswered.
• A softphone-only user with the app closedWhere somebody has no desk phone and their mobile app is not running, there is nothing registered to ring. This is increasingly common with hybrid working and is a configuration choice rather than a fault.

How to fix Q.850 cause 20

1. Try the same number from another networkIf a call from a different mobile operator also fails, the handset really is unreachable. If it connects, the problem is in the route between your system and that network, not with the subscriber.
2. Confirm the SIM is liveCheck your mobile account portal for the status of the line. Ceased and suspended SIMs are a frequent explanation, particularly a few weeks after somebody has left the business.
3. Check hot-desk and device registration for internal usersFor an internal extension, look at whether the user is logged into a handset and whether their softphone is registered. An absent user with no registered device produces this cause reliably.
4. Set a divert so unreachable does not mean lostConfigure an unreachable or unavailable divert to voicemail, a colleague or the main number. This turns a dead end into a message, which is what the caller actually needed.
5. Give mobile-dependent roles a fallbackWhere staff work in poor coverage, twinning the mobile with a desk extension or a softphone means the call has somewhere else to land while the handset is out of contact.

Questions about Q.850 cause 20

Related Q.850 cause values

• Q.850 cause 18No user responding
• Q.850 cause 19No answer from user (user alerted)
• Q.850 cause 27Destination out of order

Source. Q.850 cause 20 Subscriber absent is defined in ITU-T Q.850 section 6.2.7.1.15 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