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How Outbound Calls Work

When you initiate an outbound call through your SIP trunk, here is the complete flow:
1

You Trigger a Call via API

You send a request to the Bolna Call API with your agent ID, the recipient’s phone number, and the from_number (your SIP trunk phone number to use as the caller ID).
2

Bolna Resolves the Trunk

Bolna looks up the from_number in its database to find the associated SIP trunk and its gateway configuration (address, credentials, codecs).
3

SIP INVITE Sent via Asterisk

Bolna’s Asterisk/PJSIP server sends a SIP INVITE to your trunk’s gateway address (e.g., bolna-trunk.pstn.twilio.com or XXXX.zt.plivo.com) with the appropriate authentication.
4

Provider Routes to PSTN

Your SIP trunk provider receives the INVITE, authenticates the request, verifies the caller ID, and then routes the call to the recipient’s phone number on the PSTN.
5

AI Agent Handles the Conversation

Once the recipient answers the phone, Bolna’s Voice AI pipeline takes over. The AI agent speaks, listens, and responds in real-time throughout the entire conversation.

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Prerequisites

Before making outbound calls, make sure you have completed all previous steps:
  1. Created a SIP trunk on your provider (Plivo guide or Twilio guide)
  2. Registered the trunk on Bolna and confirmed is_active is true
  3. Added your phone numbers to the trunk on Bolna
  4. Created a Bolna AI agent via the Dashboard or the Agent API

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Step 1: Configure the Agent’s Telephony Provider

Before your agent can place calls through your SIP trunk, you need to tell it to use SIP trunking instead of the default telephony provider. Set telephony_provider to "sip-trunk" in your agent’s configuration: 
curl -X PATCH https://api.bolna.ai/v2/agent/<agent-id> \
  -H "Authorization: Bearer <your-bolna-api-key>" \
  -H "Content-Type: application/json" \
  -d '{
    "agent_config": {
      "telephony_provider": "sip-trunk"
    }
  }'
Setting telephony_provider to "sip-trunk" tells Bolna to:
  1. Route all outbound calls through your SIP trunk’s gateway instead of Twilio’s or Plivo’s standard API
  2. Use ulaw audio encoding (G.711 u-law) as required by SIP trunks
  3. Authenticate using the credentials stored in your trunk configuration

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Step 2: Place an Outbound Call

Use the standard Call API to place a call. The key difference from a regular Bolna call is the from_number field, which must be a phone number registered on your SIP trunk. 
curl -X POST https://api.bolna.ai/call \
  -H "Authorization: Bearer <your-bolna-api-key>" \
  -H "Content-Type: application/json" \
  -d '{
    "agent_id": "agt-xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx",
    "recipient": {
      "phone_number": "+918800001234",
      "name": "Rahul Sharma"
    },
    "from_number": "+919876543210"
  }'

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Field Reference

FieldDescription
agent_idUUID of the Bolna agent that will handle the conversation
recipient.phone_numberThe recipient’s phone number in E.164 format (+ followed by country code and number)
recipient.nameThe recipient’s name. This is available to the AI agent during the conversation for personalized interactions.
from_numberYour SIP trunk phone number to use as the caller ID. This number must be registered on your SIP trunk.
The from_number must be a phone number that you purchased on your SIP provider (Plivo, Twilio, etc.) and then added to your trunk on Bolna. If the number is not found on any trunk, Bolna will not know which gateway to route the call through and the call will fail.

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Step 3: Verify Call Status

After placing a call, you can monitor its status using the Call Status API: 
curl -X GET https://api.bolna.ai/call/<execution-id>/status \
  -H "Authorization: Bearer <your-bolna-api-key>"
You can also set up webhooks to receive real-time status updates as the call progresses through different stages (ringing, answered, completed, etc.).

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Batch Outbound Calling

For high-volume outbound campaigns where you need to call many people automatically, Bolna supports batch calling through your SIP trunk. Use the Batch API to schedule and manage large call campaigns. Key points for batch calling with SIP trunks:
  • Ensure the agent’s telephony_provider is set to "sip-trunk" as described in Step 1
  • Each call in the batch should use a from_number that is registered on your trunk
  • Monitor concurrency limits, which depend on your SIP trunk provider’s capacity and plan
For detailed batch calling instructions, see the Batch Calling guide.

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Troubleshooting Outbound Calls

If the call never rings on the recipient’s phone:
  • Check trunk status. Verify your trunk’s is_active is true:
curl -X GET https://api.bolna.ai/sip-trunks/trunks/<trunk-id> \
  -H "Authorization: Bearer <your-bolna-api-key>"
  • Verify from_number. Ensure the from_number you used in the call request is registered on the trunk.
  • Check gateway credentials. For userpass auth, verify the username and password in your trunk configuration are correct and match what you set up on your provider.
  • Check IP whitelist. For ip-based auth, ensure 13.200.45.61 is whitelisted on your provider’s side.
  • Review provider logs. Check your provider’s call logs for SIP error codes (e.g., 401 Unauthorized, 403 Forbidden, 404 Not Found).
This is almost always caused by an SRTP mismatch:
  • Disable SRTP / Secure Trunking on your SIP provider. Bolna does not support SRTP, and if it is enabled, the media negotiation fails silently.
  • Verify codec settings. Ensure ulaw is in the allow list on your Bolna trunk.
  • Check rtp_symmetric. This should be true (which is the default).
  • Check force_rport. This should be true (which is the default).
If you can hear the agent but the caller cannot, or vice versa:
  • Enable Symmetric RTP. Set rtp_symmetric to true on your Bolna trunk (this is the default).
  • Enable force_rport. Set force_rport to true (this is the default).
  • Check NAT traversal. Confirm your provider’s RTP IP ranges are reachable from Bolna’s IP 13.200.45.61.
  • Keep direct_media disabled. Ensure direct_media is false (the default). Enabling it bypasses Bolna’s media server, which often causes one-way audio.
Some carriers require the + prefix on dialed numbers (E.164 format), while others reject it.
  • Twilio requires the + prefix. Keep outbound_leading_plus_enabled as true (default).
  • Plivo generally accepts both formats.
  • Other providers: Check your provider’s documentation for their preferred number format.
You can update this setting per-trunk:
curl -X PATCH https://api.bolna.ai/sip-trunks/trunks/<trunk-id> \
  -H "Authorization: Bearer <your-bolna-api-key>" \
  -H "Content-Type: application/json" \
  -d '{ "outbound_leading_plus_enabled": false }'
If the provider rejects the call because of an invalid caller ID:
  • Twilio: The from_number must be either a Twilio DID number on your account or a verified Caller ID number.
  • Plivo: The from_number must be a number in your Plivo account.
  • Ensure the number format matches what your provider expects. E.164 (with +) is recommended for all providers.

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Next Steps

Receive Inbound Calls

Route incoming calls to your Bolna AI agents

Batch Calling

Run high-volume outbound AI call campaigns

Monitor Call Status

Track call status and outcomes in real-time

SIP Trunk API Reference

Full API reference for managing trunks and numbers