Short duration calls (SDC) are answered calls that last only a few seconds. A high share of them can be normal for some traffic, such as dialers, but on other routes it signals dropped calls, bad audio, false answers or fraud, which is why carriers both monitor SDC and often limit it in their terms.
What counts as a short call
There is no single standard threshold. Carriers define SDC in their terms, usually as answered calls shorter than a fixed number of seconds, often somewhere between 6 and 30 seconds. The SDC ratio is the share of answered calls below that threshold:
SDC % = answered calls under the threshold / all answered calls x 100
Check the definition in your agreement before comparing numbers between carriers, because a 6 second and a 30 second threshold produce very different percentages.
Why carriers care about SDC
Short calls cost the network nearly as much as long ones: each call needs signalling, a channel and switching capacity, but generates very little billable time. Termination costs at many destinations also include per call charges or minimum durations. A route that carries many short calls is therefore expensive to run, which is why some carriers:
- accept only traffic with an SDC below an agreed ratio;
- apply a surcharge per short call or a minimum billed duration;
- move high SDC traffic to separate, dialer friendly routes, as explained in dialer traffic and VoIP routes.
Normal causes
- Dialer and call center traffic: people hang up quickly on sales calls; answering machine detection drops calls after a few seconds.
- Wrong numbers: callers realise they dialled the wrong person.
- Voicemail checks and callbacks: short by nature.
The traffic types and how routes treat them are covered in VoIP traffic types.
Causes that point to a problem
| Pattern | What to check |
|---|---|
| SDC jumps on one route while the same traffic is fine elsewhere | Audio problems on that route: one-way audio, silence, echo |
| Many answered calls of 1 to 3 seconds with no conversation | False answer supervision, see how to spot FAS |
| Calls cut at the same duration every time | Signalling or timer issues, see one-way audio and dropped calls |
| Bursts of short inbound calls from international numbers | Possible Wangiri activity |
| Short calls to a few unusual number ranges | Test calls before revenue share fraud; see IRSF |
How to monitor and reduce SDC
- Calculate SDC per route, per destination and per customer every day, from your CDRs. A single blended number hides the route that causes the problem.
- Set an alert for sudden changes, not just an absolute level: a jump from 10% to 25% on one route deserves a look even if 25% is normal for another customer.
- Look at the duration histogram. Real hang ups spread across many durations; technical problems pile up at one value.
- For dialer traffic, tune answering machine detection and pacing, and buy routes that accept dialer traffic openly instead of hiding it in conversational routes.
- Read SDC together with ACD, ASR and NER, as described in monitoring route quality with CDRs.
When you request a route, state your expected SDC and traffic type: it lets us match you with a carrier that accepts it, which avoids blocked traffic later. VoIPMatcher sources VoIP termination for conversational, call center and dialer traffic. Request a VoIP route.
Frequently asked questions
What is a good SDC ratio?
It depends on the threshold and the traffic type. Conversational traffic should show a low share of very short calls, while dialer traffic naturally shows many more. Compare each route with its own history and with other routes carrying the same traffic, and check your agreement, since carriers set the threshold differently, often between 6 and 30 seconds.
Are short calls billed?
Yes, answered short calls are billed under the agreed billing increment. On 60/60 billing a 5 second call costs a full minute, on 6/6 it costs 6 seconds and on 1/1 it costs 5 seconds. Some carriers also add a surcharge per short call or a minimum billed duration on traffic with many short calls.
How does answering machine detection affect short calls?
Answering machine detection listens to the first seconds of an answered call and hangs up when it detects voicemail, so every machine-answered call becomes a short answered call. That raises SDC on dialer traffic. Tuning detection and pacing reduces it, and routes that openly accept dialer traffic expect this pattern.
What is the difference between ACD and SDC?
ACD is the average length of answered calls; SDC is the share of answered calls below a short threshold. They often move together but answer different questions: ACD can look acceptable while a large group of calls is cut after a few seconds. Read SDC per route alongside ACD, ASR and NER.
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