Concurrency limits

Concurrency is how many calls can be live at the same moment. It is a different thing from your monthly quota: a quota is how much you may use over a month, concurrency is how much you may use right now.

Three separate ceilings apply to every call. A call has to clear all of the ones that apply to it:

CeilingApplies toDefaultWho sets it
Per numberPSTN calls (in and out) and softphone calls, counted on each phone number2Platform admin
Per workspaceEvery live call — phone, web and softphone togetherunset (∞)You (owner/admin)
BrowserWeb calls only — widget, test calls, SDK sessions50Platform admin

The one that surprises people is the per-number cap. It is not a per-workspace limit. A number with a cap of 2 carries two simultaneous calls no matter how many numbers, agents or credits you have. If you want 20 calls at once you need either a higher per-number cap or more numbers — or a campaign, which paces the calls for you.

Setting the per-workspace cap

This is the one you control yourself.

  1. Go to Usage in the sidebar.
  2. Click Edit limits (top-right).
  3. Set Max concurrent calls — the number of simultaneous live calls your workspace may have.
  4. Leave it blank for unlimited. A value of 0 hard-pauses all calling.
  5. Make sure Enforce these limits is on. With it off the meters still count but nothing is blocked (the panel shows an Observe-only badge).
  6. Click Save limits.

The Concurrent now meter on the Usage page shows live usage against this cap.

Requires owner or admin. See Usage & billing for the monthly quotas that live in the same dialog.

Setting the per-number and browser caps

These are platform-admin settings, because they protect the carrier and the platform, not just your spend. If you are on a hosted plan, ask your platform admin; the steps below are for the admin console.

Change the default for every org

  1. Open the admin console → Platform fee.
  2. Find Default concurrency caps.
  3. Set:
    • Browser (per org) — max simultaneous web calls per organization.
    • Telephony (per number) — max simultaneous PSTN/softphone calls on each number.
  4. Save.

These apply to every organization that has no override of its own.

Override for one org

  1. Open the admin console → Billing accounts.
  2. Find the organization's row and click the concurrency cell (it reads default, or e.g. 50 web / 4 per # when overridden).
  3. Enter the browser cap, then the per-number cap.
  4. Leave either empty to fall back to the platform default.

Raise the per-number cap only as far as the carrier and the number's own channel count actually allow. Setting 20 on a number the carrier will only carry 4 calls on does not create capacity — it just moves the refusal from us to the carrier, where it arrives as a failed call instead of a clean 429.

What happens when a limit is hit

The call is refused before the carrier is contacted — never silently dropped, and never billed. You get 429 Too Many Requests: its scope names the ceiling you hit — number, org or browser — and its Retry-After says how long to wait. That wait is worked out from your place in the queue, so honour it rather than retrying on a fixed timer of your own.

For the response body, its fields and a retry example, see Rate limits & quotas.

Don't fan out parallel dials

The single-dial API places one call. Firing it 200 times in parallel does not produce 200 calls — it produces however many your caps allow and a pile of 429s, because the ceiling is a property of the phone number, not of how hard you ask.

For anything bulk, use a campaign: you hand over the whole list once, and the platform paces the dialing within your caps, retries no-answers, respects quiet hours and reports every outcome back.

Which cap is stopping me?

SymptomCauseFix
scope: "number" on one number while others workThat number is fullMore numbers, a higher per-number cap, or a campaign
scope: "org" no matter which numberWorkspace capUsage → Edit limits
scope: "browser" on widget/test calls onlyBrowser capAsk your platform admin
403, not 429Not concurrency — a monthly quota, a spend cap, or a suspended accountUsage & billing
Campaign dials far fewer at once than its concurrency settingThe campaign's own concurrency is capped again by the per-number ceilingSpread the campaign across more numbers

A campaign has its own concurrency setting too. It is an upper bound on that campaign, not an exemption: a campaign set to 10 on a single number with a per-number cap of 2 still runs 2 at a time.

How the limits behave in practice

  • Counted the moment a call is admitted, not when it connects — so a burst of simultaneous requests is refused correctly rather than all being let through.
  • Freed the moment a call ends, including calls nobody answered.
  • Shared across servers, so the ceiling is the same no matter which server takes your request.
  • Never fail closed. If the tracking layer is unavailable the platform keeps calling rather than stopping your business; enforcement resumes by itself.