Make outbound calls with Retell agents

Overview

Make outbound calls with your Retell agents: bind an agent to a phone number, then trigger calls from the dashboard or the API.

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

Configuration options

Setting	Purpose	Use case
Inbound Agent	Handles incoming calls to this number	Customer support, callbacks
Outbound Agent	Used when making calls from this number	Sales 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!

A/B testing

See A/B Testing.

Step 2: Make outbound calls

International calling restrictions

Retell-purchased numbers: Retell supports calling to 15 countries.Imported numbers: International calling depends on your telephony provider’s settings.

Call parameters

When making outbound calls (v2), these parameters are supported:

Parameter	Type	Example	Description
`from_number`	string (E.164)	`+14157774444`	Your Retell-managed or imported number.
`to_number`	string (E.164)	`+12137774445`	Destination number.
`override_agent_id`	string	`agent_abc123`	Override the agent used for this call (optional).
`override_agent_version`	integer	`1`	Version of the override agent; defaults to latest if omitted.
`agent_override`	object	See below	Per-call partial overrides for agent/response engine (optional).
`metadata`	object	`{ "customer_id": "cust_123" }`	Free-form metadata stored with the call (size limit applies).
`retell_llm_dynamic_variables`	object	`{ "name": "John" }`	Key–value strings injected into prompts/tools (optional).
`custom_sip_headers`	object	`{ "X-Call-ID": "123" }`	Outbound SIP headers forwarded to your provider (optional).
`ignore_e164_validation`	boolean	`false`	Only for custom telephony. Bypass E.164 validation for special routing.

Agent overrides let you adjust per-call behavior without changing the saved agent. See “Agent Overrides” in the Create Phone Call API for supported fields and examples.

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
  // Optional: per-call agent selection and overrides
  override_agent_id: 'agent_abc123',
  override_agent_version: 0, // or omit to use latest
  agent_override: {
    agent: {
      voice_speed: 1.1,
      enable_backchannel: true,
    },
    // retell_llm or conversation_flow overrides are also supported
  },

  retell_llm_dynamic_variables: { // dynamic variables (optional, string values only)
    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);

# Initiate an outbound call using the newly created agent
call = client.call.create_phone_call(
    from_number="+14157774444", # replace with the number you purchased
    to_number="+12137774445",  # replace with the number you want to call
    # Optional: per-call agent selection and overrides
    override_agent_id="agent_abc123",
    override_agent_version=0,  # or omit to use latest
    agent_override={
      "agent": {
        "voice_speed": 1.1,
        "enable_backchannel": True,
      }
    },

    retell_llm_dynamic_variables={ # dynamic variables (optional, string values only)
      "name": "John Doe",
      "blood_group": "B+"
    },
    custom_sip_headers={ # replace with custom sip headers you want to send (optional)
      "X-Custom-Header": "Custom Value"
    }
)
print(call)

Step 3: Configure CPS (calls per second)

Understanding CPS limits

CPS (calls per second) controls how many outbound calls you can start each second.

Default limits and scaling

Provider	Default CPS	Maximum CPS	Notes
Twilio	1	5	Changes take up to 10 minutes
Telnyx	1	16	Instant updates
Custom Telephony	1	150	Check your custom provider’s SIP trunk capacity before increasing this limit

Important considerations

Throttling protection: Exceeding limits results in rejected calls
Gradual scaling: Start low and increase based on actual needs
Provider limits: Your telephony provider may have additional restrictions
Cost impact: Higher CPS may increase telephony costs
Concurrency: CPS controls how fast calls start; the number that can run at once is capped separately by your concurrency limit

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

Community Templates: Real-world outbound calling examples
Batch calling: For high-volume campaigns
Handle voicemail and IVR: Detect voicemail or IVR menus on outbound calls
Capture DTMF input: Collect keypad digits from the caller
Debug outbound calls: Troubleshoot connection issues
Webhook setup: Configure real-time notifications

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Other Topics

Make outbound calls

Overview

Prerequisites

Step 1: Bind agents to phone numbers

Configuration options

Flexible agent assignment

A/B testing

Step 2: Make outbound calls

International calling restrictions

Call parameters

API implementation

Step 3: Configure CPS (calls per second)

Understanding CPS limits

Default limits and scaling

Important considerations

Best practices for high-volume calling

Step 4: Monitor call details

Available monitoring methods

1. API polling

2. Real-time webhooks

Additional resources

​Overview

​Prerequisites

​Step 1: Bind agents to phone numbers

​Configuration options

​Flexible agent assignment

​A/B testing

​Step 2: Make outbound calls

​International calling restrictions

​Call parameters

​API implementation

​Step 3: Configure CPS (calls per second)

​Understanding CPS limits

​Default limits and scaling

​Important considerations

​Best practices for high-volume calling

​Step 4: Monitor call details

​Available monitoring methods

​1. API polling

​2. Real-time webhooks

​Additional resources

Overview

Prerequisites

Step 1: Bind agents to phone numbers

Configuration options

Flexible agent assignment

A/B testing

Step 2: Make outbound calls

International calling restrictions

Call parameters

API implementation

Step 3: Configure CPS (calls per second)

Understanding CPS limits

Default limits and scaling

Important considerations

Best practices for high-volume calling

Step 4: Monitor call details

Available monitoring methods

1. API polling

2. Real-time webhooks

Additional resources