Q.850 cause 17 User busy

• Normal class
• Sent by The called handset, its PBX, or the terminating network
• Common

What Q.850 cause 17 User busy actually means

The call reached the destination successfully and was refused because a busy condition was found. The specification allows either the called equipment or the network to generate it, and notes that where the user determines the busy condition the equipment is otherwise compatible with the call — in other words, this is a capacity limit at the endpoint, not an incompatibility. Under RFC 3398 it becomes SIP 486 Busy Here. Keep it separate in your mind from cause 34, which is about circuits running out rather than a person being occupied.

This cause is used to indicate that the called party is unable to accept another call because the user busy condition has been encountered. This cause value may be generated by the called user or by the network. In the case of user determine user busy, it is noted that the user equipment is compatible with the call.

— ITU-T Q.850 section 6.2.7.1.12

The SIP equivalent of Q.850 cause 17

A gateway translating this cause into SIP reports it as SIP 486 Busy Here. The mapping is defined in RFC 3398 section 7.2.4.1.

What causes Q.850 cause 17 on a business phone system

• The extension is on a call with call waiting offThe default configuration on many business systems, and deliberately so — being interrupted mid-conversation is not always wanted. The second caller gets engaged rather than a beep.
• Every call appearance on the handset is occupiedPhones support a fixed number of simultaneous legs. Calls parked on hold still consume an appearance, so a user who feels they are on one call may in fact be holding three and unable to receive a fourth.
• Every member of a hunt group is engagedA group that returns busy when all members are busy will surface as cause 17 to the caller. This is the group behaving as configured, but it is rarely the experience you want on a main number.
• An analogue device behind an ATAA single analogue line can carry one call. A fax machine, door entry panel or card terminal on an ATA port will return busy for any second call, and there is no way to configure around the physical limitation.
• Do not disturb presented as busyBehaviour varies by platform: some systems report DND as busy, others as rejected or unavailable. If busy responses appear for someone who is demonstrably free, check whether DND has been left on.

How to fix Q.850 cause 17

1. Decide whether a second call should be able to arriveEnabling call waiting is the direct answer, but consider whether the user actually wants it. For roles where the current conversation matters most, a busy-forward is the better outcome.
2. Set a busy-forward destinationPoint calls that arrive while an extension is engaged to voicemail, a colleague or a group. Nobody hears engaged tone, and you have a record of who tried to get through.
3. Move shared numbers onto a queueA main business number should not terminate on a single extension. A queue or hunt group means one busy person never blocks the number, and it gives you the reporting to size the team properly.
4. Check for do not disturb before escalatingA single user reporting that callers always get engaged is very often DND left on from the previous day. It takes seconds to check and saves a pointless trunk investigation.
5. Rule out a trunk capacity limit masquerading as busyIf busy responses appear across many extensions at the same times of day, the constraint is trunk channels rather than individual phones. That is cause 34 territory and needs a different fix.

Questions about Q.850 cause 17

Related Q.850 cause values

• Q.850 cause 19No answer from user (user alerted)
• Q.850 cause 21Call rejected
• Q.850 cause 34No circuit/channel available

Source. Q.850 cause 17 User busy is defined in ITU-T Q.850 section 6.2.7.1.12 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