Call Concurrency noun
The maximum number of simultaneous calls that your account can have active at any given moment.

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🔑 TL;DR

Plan TypeConcurrency CapPriority Access
Free / PAYGO5 calls❌
ProNo hard cap*❌
ScaleNo hard cap*✅ Up to 100
*Still subject to infra limits under extreme load.

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Call Concurrency in Ultravox Realtime

Ultravox works differently than most other voice AI platforms. Unlike other providers, we manage our own GPUs running the Ultravox model. This gives us granular control over the performance of our system. When you create a voice AI call on Ultravox Realtime, we dedicate GPU resources to your call for its entire lifespan. This allows us to keep your latency low and consistent, regardless of conversation length or traffic to our system. In contrast, most voice AI providers rely on shared inference pools provided by LLM vendors. These systems queue each request and dynamically assign them to available GPUs, often using microbatching for efficiency. This leads to variable latency during the call and makes scaling reliably challenging. If you’re on a paid subscription plan, there are no hard caps on concurrency. If you’re on the pay as you go model, there is a hard cap of 5 concurrent calls. Our goal is to dynamically match compute with demand, but during periods of extremely high load, it’s possible that we might not have compute available to serve your call. When that happens, we’ll issue an HTTP 429 status code (“Too Many Request”) so you know to try again after a short wait. This system is designed to help customers scale without having to overpay for concurrency. Most customers don’t need the same amount of concurrency 24 hours a day. Ultravox Realtime is designed to scale with you, and we balance load with 429s to keep the system fair for everyone. More on how to handle 429s below. For customers that have high, sustained load, we offer priority call concurrency on our Scale plan. We also offer dedicated capacity as part of our enterprise plans.

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Unbounded Concurrency

All accounts on a paid subscription enjoy no hard caps on call concurrency. This means not having to worry about paying for concurrency slots that are only needed occasionally for spikes in traffic. If the system is under load and we are unable to fulfill a request to create a call, you may receive a 429 “Too Many Requests” response. See below for more on how to properly handle this.

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Priority Concurrency

Accounts on the Scale plan enjoy priority in the system for up to 100 concurrent calls (need even more? contact us to discuss our enterprise agreements). This provides peace of mind for high impact inbound calls and ensuring that your most critical voice interactions are always available, even during peak system demand. If we are unable to create a call when an account is below its allotted priority call count, we will return a 503 “Service Unavailable” error. 429s are used if we are unable to fulfill new call requests above an account’s priority call limit. See below for more on how to properly handle 429s and 503s in your code.

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Hard Concurrency Caps

Ultravox Realtime accounts default to a hard cap on call concurrency. Any account not on a monthly or annual subscription is limited to five concurrent calls. Any attempt to create additional calls above the hard cap will result in an immediate HTTP 429 “Too Many Requests” response, allowing you to implement proper retry logic and queue management in your application. See below for more on how to properly handle this.

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Managing Concurrency

If you encounter concurrency limits, proper handling in your application ensures a smooth user experience and optimal resource utilization. The key is implementing robust retry logic and monitoring your concurrent call usage.

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Example: Hitting the Cap

Let’s consider an example where a pay-as-you-go account attempts to create multiple Ultravox calls over a short time:
TimeActive CallsNew RequestStatusConcurrent Count
0s-Create Call 1✅ Success1/5
2sCall 1Create Call 2✅ Success2/5
3sCall 1,2Create Call 3✅ Success3/5
4sCall 1,2,3Create Call 4✅ Success4/5
5sCall 1,2,3,4Create Call 5✅ Success5/5 (At Limit)
6sCall 1,2,3,4,5Create Call 6❌ HTTP 4295/5 (Rejected)
7sCall 2,3,4,5Create Call 7❌ HTTP 4295/5 (Rejected)
8sCall 2,3,5Create Call 8✅ Success4/5

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Keeping the Pipe Full

When you receive a 429 or 503 response, it’s important to use Retry-After header to implement a proper retry strategy and avoid overwhelming the system. The Retry-After header is used to provide the number of seconds to wait before making any additional new requests. Here’s an example of how to do that with an exponential backoff + retry handling:
Creating Calls with Retry Logic
async function createCallWithRetry(callConfig, maxRetries = 3) {
  for (let attempt = 0; attempt < maxRetries; attempt++) {
    try {
      const response = await fetch('https://api.ultravox.ai/api/calls', {
        method: 'POST',
        headers: { 'Content-Type': 'application/json' },
        body: JSON.stringify(callConfig)
      });

      const retryAfter = response.headers.get('Retry-After');
      
      if (retryAfter) {
        const delay = parseInt(retryAfter) * 1000;
        console.log(`Retry-After header found. Retrying in ${delay/1000}s...`);
        await new Promise(resolve => setTimeout(resolve, delay));
        continue;
      }

      if (response.ok) {
        return await response.json();
      }
      
      throw new Error(`HTTP ${response.status}: ${response.statusText}`);
    } catch (error) {
      if (attempt === maxRetries - 1) throw error;
      await new Promise(resolve => setTimeout(resolve, Math.pow(2, attempt) * 1000));
    }
  }
}

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Outbound Call Scheduler (Expected July 2025)

Set it and forget it. You don’t have to sweat the load or keep track of 429s. Leave that to us. Available to all accounts with a paid subscription. You Provide
  • Time window (e.g. tomorrow between 8am-5pm)
  • List of destination phone numbers
  • Desired agent to use for the calls
We Handle
  • Queueing of all calls
  • Load balancing
  • Retries and 429 handling (these are fully abstracted)
Use Cases
  • Marketing call campaigns
  • Proactive customer service follow-ups
  • High-volume appointment reminders