Concurrency
Concurrency refers to the number of simultaneous active voice calls that can be handled by your system at any given moment. For example, if 15 users are engaged in voice calls with your agents at the same time, that counts as 15 concurrent calls. Concurrency limits apply per workspace, not per account. Each workspace has its own quota, burst settings, and reserved inbound capacity, and traffic in one workspace does not consume slots in another. Pay-As-You-Go workspaces are allocated a quota of 20 concurrent calls by default. If your operational needs require more concurrency, you can adjust your limit from the dashboard. See Manage limits in the dashboard below. You can check your current number of concurrent calls in the dashboard.- Handling Multiple Calls per Agent: You don’t need to create multiple agents to manage multiple calls concurrently. Each agent within your plan is capable of handling an unlimited number of calls, provided that the total concurrency remains within your designated quota. This means you can efficiently manage your workload without unnecessary agent duplication.
Reserved Inbound Concurrency
Reserved inbound concurrency protects inbound calls from being crowded out by outbound traffic. Whenreserved_inbound_concurrency is configured, outbound calls can use at most your concurrency
limit minus the reserved amount. Inbound calls can still use the full concurrency limit when capacity
is available.
For example, if your concurrency limit is 100 and reserved inbound concurrency is 20:
- Outbound calls can use up to 80 slots.
- Inbound calls can use the reserved 20 slots, plus any other available slots up to the full 100.
Inbound Queue and Fallback
When inbound call traffic reaches your concurrency limit, Retell briefly keeps new inbound calls waiting for an available slot. If a slot opens, the inbound call proceeds. If no slot opens after about 40 seconds, Retell handles the call as follows:- If the phone number has a
fallback_numberconfigured, Retell transfers the caller to that number. - If there is no fallback number, or the fallback transfer fails, the call ends with
concurrency_limit_reached. - If the fallback transfer succeeds, the Retell call record ends with
no_concurrency_fallback.
Concurrency Burst
Concurrency Burst allows you to temporarily exceed your standard concurrency limit during peak demand periods. When enabled, calls that would normally be rejected due to hitting your concurrency limit will instead be allowed to proceed with an additional surcharge.How It Works
When concurrency burst is enabled:- Normal calls: Calls within your standard concurrency limit proceed as usual with no additional charges
- Burst calls: Calls that exceed your normal limit (but stay within the burst limit) will proceed with an additional $0.10/min surcharge applied to the entire call duration
Burst Limit Calculation
Your burst limit is calculated as the lower of:- 3× your concurrency limit, OR
- Your concurrency limit + 300
- If your limit is 50, burst allows up to 150 concurrent calls (3 × 50 = 150)
- If your limit is 200, burst allows up to 500 concurrent calls (200 + 300 = 500, which is less than 3 × 200 = 600)
Enabling Concurrency Burst
You can enable or disable concurrency burst from the Settings > Limits page in your dashboard.
Pricing
| Call Type | Additional Cost |
|---|---|
| Normal (within standard limit) | No additional charge |
| Burst (above standard limit) | $0.10/min for the entire call duration |
The burst surcharge applies to the entire duration of any call that started while in burst mode, not just the portion of time spent above the normal limit.
Use Cases
Concurrency burst is ideal for:- Unpredictable traffic spikes: Handle sudden increases in call volume without rejected calls
- Campaign launches: Support higher-than-normal call volumes during marketing campaigns
- Seasonal peaks: Manage increased demand during busy periods without permanently upgrading your concurrency limit
Manage limits in the dashboard
You can view and adjust your concurrency and CPS limits from the Settings > Limits page.
Adjusting concurrency and CPS limits is available to the Admin and Developer roles. Reserving inbound capacity and toggling concurrency burst change workspace settings and require the Admin role. See Access Control for details.
Adjust your concurrency limit
On the Concurrent Calls Limit card, click Adjust Concurrency. The dialog shows your current limit and how high you can go.
Reserve inbound capacity
On the same card, click Reserve Inbound Capacity to set how many slots are held for inbound calls (see Reserved Inbound Concurrency above). The remainder is available to outbound and web calls.
Adjust CPS (calls per second)
CPS is how quickly new calls can be started, set per telephony path. The Limits page has a card for Telnyx CPS, Twilio CPS, and Custom Telephony CPS. Click Adjust Limit on a card to change that provider’s CPS; each has its own allowed range. Custom Telephony CPS scales with your concurrency, so a higher CPS there may require more concurrency.
Estimate values with the calculator
If you’re unsure what to set, open the calculator from the top of the Limits page. Enter your inbound and outbound traffic (calls per busy hour, average durations, and pickup rate) and it returns a recommended concurrency, inbound reservation, and CPS, each with headroom for spikes. These are suggestions; you still apply them with the cards above.