​
Overview

This guide covers how to make outbound calls with your Retell agents. Before proceeding, ensure you have:
  • Created and configured an agent
  • Purchased or imported phone numbers
  • Set up your API credentials

​
Prerequisites

  • Phone Number: A Retell-managed or imported number
  • Agent: A configured agent ready for outbound calls
  • API Key: Your Retell API key for authentication

​
Step 1: Bind Agents to Phone Numbers

Before making calls, you must assign agents to your phone numbers. This configuration determines how your number handles both inbound and outbound calls.

​
Configuration Options

SettingPurposeUse Case
Inbound AgentHandles incoming calls to this numberCustomer support, callbacks
Outbound AgentUsed when making calls from this numberSales outreach, notifications

​
Flexible Agent Assignment

  • Different agents: Use specialized agents for inbound vs outbound
  • Outbound only: Leave inbound agent unset to prevent callbacks
  • Inbound only: Configure only inbound agent for receive-only numbers
Phone number configuration showing inbound and outbound agent assignment
After binding an inbound agent, your number is immediately ready to receive calls. Test it by calling the number!

​
Step 2: Make Outbound Calls

​
International Calling Restrictions

Retell-purchased numbers: Currently limited to domestic calls only. Imported numbers: International calling depends on your telephony provider’s settings.

​
Call Parameters

When making outbound calls, you’ll need:
ParameterFormatExampleDescription
from_numberE.164+14157774444Your Retell number
to_numberE.164+12137774445Destination number
dynamic_variablesObject{name: "John"}Optional personalization
custom_sip_headersObject{"X-Call-ID": "123"}Optional SIP headers
Outbound call configuration interface

​
API Implementation

For complete parameter documentation, see Create Phone Call API Reference. 
const registerCallResponse = await retell.call.createPhoneCall({
  from_number: '+14157774444', // replace with the number you purchased
  to_number: '+12137774445',  // replace with the number you want to call
  retell_llm_dynamic_variables: { // replace with the dynamic variables you want to send (optional)
    name: 'John Doe',
    blood_group: 'B+'
  },  
  custom_sip_headers: { // replace with custom sip headers you want to send (optional)
    X-Custom-Header: 'Custom Value'
  }
});
console.log(registerCallResponse);

​
Step 3: Configure CPS (Calls Per Second)

​
Understanding CPS Limits

CPS (Calls Per Second) controls how many outbound calls you can initiate per second. This prevents system overload and ensures call quality.

​
Default Limits & Scaling

ProviderDefault CPSMaximum CPSNotes
Twilio15Changes take up to 10 minutes
Telnyx116Instant updates
Custom Telephony1Concurrency ÷ 20Based on your concurrency limit

​
Important Considerations

  1. Throttling Protection: Exceeding limits results in rejected calls
  2. Gradual Scaling: Start low and increase based on actual needs
  3. Provider Limits: Your telephony provider may have additional restrictions
  4. Cost Impact: Higher CPS may increase telephony costs

​
Best Practices for High-Volume Calling

Implement retry logic with exponential backoff to handle throttling gracefully:
// Example retry logic
const maxRetries = 3;
let retryDelay = 1000; // Start with 1 second

for (let i = 0; i < maxRetries; i++) {
  try {
    await makeCall();
    break;
  } catch (error) {
    if (error.code === 'rate_limited') {
      await sleep(retryDelay);
      retryDelay *= 2; // Exponential backoff
    }
  }
}

​
Step 4: Monitor Call Details

​
Available Monitoring Methods

​
1. API Polling

Use Get Call API to retrieve:
  • Full transcript
  • Call recording
  • Latency metrics
  • Function call logs
  • Call duration and status

​
2. Real-time Webhooks

Set up webhooks to receive instant notifications for:
  • Call Started: When the call connects
  • Call Ended: Final status and duration
  • Call Analyzed: Transcript and analysis ready
  • Call Failed: Error details and reasons
Webhooks provide real-time updates without polling, making them ideal for production systems.
Triggering Outbound Calls (Using Make.com)

​
Additional Resources