Ten customers call a Luxembourg retailer in the same minute. The shop has ten employees, but only three are free to answer. What happens to the other seven depends on how many calls the phone connection can accept, how many people can wait in the queue and how quickly staff become free.
Where do the ten incoming calls go?
Imagine the shop can accept eight calls at once and has room for four callers to wait. All ten calls arrive before any of the first three conversations end. The test starts with an empty queue and no calls already in progress.
- Calls 1–3 reach staff. Each available employee takes one call. The next caller needs a place to wait.
- Calls 4–7 enter the queue. Four callers wait. If someone hangs up or a member of staff answers, a place opens up. If a caller waits past the time limit, the system follows its timeout rule.
- Call 8 reaches a full queue. The connection can still accept it, so the phone system applies its overflow rule. In this example, it sends the caller to voicemail. Other setups may send the call to a backup team or end it.
- Calls 9–10 reach a full connection. If call 8 is still in voicemail and the other seven calls are active, both new callers hit the eight-call limit. They may hear a busy tone or a network message, or reach an alternate destination if the provider has set one up.
A SIP trunk is the service that connects some business phone systems to the public phone network. Ask your provider or installer whether incoming and outgoing calls share a limit and whether callers in the menu, queue or voicemail count against it. For example, 3CX documents a shared incoming and outgoing limit per trunk. A hosted phone service may apply limits elsewhere, so ask which limit applies to your number.
Check the rules for a full queue, a long wait or no available staff separately. If you need to brief a colleague, share the PBX.lu glossary of call queues and routing.
What happened in your busiest 15 minutes?
Start with the call logs from last Black Friday or a recent promotion. Find the 15-minute window with the most incoming attempts, then look at how many calls overlapped. Twenty calls spread across 15 minutes can put less pressure on the connection than ten arriving together.
- Count calls in progress at the same time. If you have start and end times, include calls already active at the start of the window and outgoing calls that share the limit. For incoming attempts, count each customer call once. For capacity, use the provider’s channel counts if forwarding or transfers create extra connections. Ask the provider for peak usage if your own logs lack enough detail.
- Compare talk time with wait time. Long stock checks or order lookups can keep staff busy. Check how long people waited to speak to someone, including those who gave up if your report shows that separately.
- Separate hang-ups, timeouts and rejected calls. A caller who hangs up while waiting had a different experience from one whose call never reached the queue. Microsoft’s queue reporting guide shows how these outcomes can be reported separately. Ask your provider for records of rejected calls if your phone system cannot see them.
How do you test capacity before Black Friday?
Agree a short, quiet test window with the person who manages your phone system. If you test outside opening hours, make sure the test follows the same routing customers will hear during the sale. Use a test number with the same routing and capacity limits where possible. Stop if the test interferes with real calls.
- Call from external phones. Keep the first calls active with staff, then add calls one at a time so some callers are waiting. Ask the provider how to check the connection limit separately if the queue fills first.
- Write down what each new caller hears and how long they wait. Test what happens when the queue fills, when a caller reaches the wait limit and when nobody is signed in to answer.
- While calls are still active, ask a colleague to call an external test phone back. Check whether it rings and whether both people can hear each other.
- Compare the caller’s experience with the logs. If you have separate French and German menus, check each route and confirm that the backup team can answer in the chosen language.
What should you change after the test?
What you observed | What to do next |
Calls fail before the greeting | Give the provider the test times and ask them to check the incoming route and simultaneous-call limit. |
The queue fills or callers give up | Put more staff on phones at the peak, shorten avoidable delays and check where calls go when the queue fills. |
Callbacks fail during the rush | Ask whether incoming and outgoing calls share capacity and test the callback route again after a change. |
Stock and delivery questions take up most of the team’s time | Put stock availability, collection times and delivery updates where customers can find them on product and order pages. |
Repeat the test after making a change. If calls reach the queue but customers still give up, start with the staff schedule and how calls reach available staff. Extra lines can let more callers through, but they cannot clear the queue any faster on their own.