Configure blind transfer modes

Blind transfer sends the active call to a destination without speaking to the destination first. Use it when the destination can accept the call immediately and does not need a spoken introduction.

This guide starts with an existing transfer call tool. To create the tool, configure destinations, and add it to an assistant, follow Set up the Transfer Call tool.

Both blind modes transfer the caller immediately. The summary variant adds context for a Session Initiation Protocol (SIP) system without speaking that context aloud.

Dashboard mode	API value	Destination experience
Blind transfer	`blind-transfer`	Receives the call without a message or summary
Blind transfer with summary in SIP header	`blind-transfer-add-summary-to-sip-header`	Receives the call with a generated summary in the `X-Transfer-Summary` SIP header

Blind transfer is the default. If transferPlan is omitted, Vapi uses blind-transfer.

Neither blind mode waits for an operator to accept the call. The assistant-forwarded-call ended reason means the transfer started, not that the destination answered.

What the caller and destination hear

The caller hears the transfer message. See Configure transfer messages.
The original assistant leaves the call, and the transfer starts.
The destination rings. When it answers, the caller and destination are connected. Neither one hears an introduction.

How the transfer reaches the destination depends on the phone provider:

Phone provider	How the transfer reaches the destination
Twilio	Twilio dials a phone-number destination. For a SIP destination, it sends a SIP REFER, or dials when `sipVerb` is `dial`.
Vapi numbers	Vapi dials the destination, whatever `sipVerb` is set to.
Bring-your-own SIP trunk	Vapi sends a SIP REFER by default. See Choose a SIP verb.
Telnyx	Telnyx transfers the call to the destination.
Vonage	Vonage connects a phone-number destination. Vonage can’t transfer to a SIP address.

If voicemail answers, the transfer counts as answered and the caller is connected to the voicemail.

With blind-transfer-add-summary-to-sip-header, the summary goes in the X-Transfer-Summary SIP header and is never spoken. Only Vapi numbers and bring-your-own SIP trunks send the header.

If the transfer fails

The caller doesn’t return to the original assistant by default. What happens depends on the phone provider.

Twilio, Telnyx, and Vonage: Vapi leaves the call when the transfer starts, so a busy, unanswered, or failed destination can’t return the caller to the assistant. The call ends with assistant-forwarded-call. Check the provider’s call record to see whether the destination answered.

Vapi numbers and bring-your-own SIP trunks: Vapi watches the transfer and records the outcome:

Outcome	`endedReason`
The destination is busy	`call.forwarding.operator-busy`
The destination doesn’t answer	`call.forwarding.no-answer`
The destination rejects the call or the transfer fails	`call.in-progress.error-transfer-failed`
Vapi receives no final outcome	`assistant-forwarded-call`

To return the caller to the assistant after a failed transfer, add a fallbackPlan to the destination’s transferPlan. The assistant says fallbackPlan.message, which defaults to Could not transfer the call. Then:

With endCallEnabled set to false, the assistant keeps helping the caller.
With endCallEnabled set to true or not set, the call ends.

With the bye verb, Vapi can recover only from a failure it detects before it leaves the call. See Return the caller to the assistant after a failed transfer.

Update the transfer plan on each destination that should use blind transfer.

Dashboard

curl

Open the transfer destination

In the Vapi Dashboard, open Tools, select the transfer call tool, and expand Destinations. Select the phone-number or SIP destination you want to configure.

Select the transfer mode

Expand Transfer Plan. Under Transfer mode, select Blind transfer or Blind transfer with summary in SIP header.

Transfer Mode menu showing the blind and warm transfer options — Select a blind transfer mode in the Dashboard

Choose the SIP verb

Under SIP Verb, select the behavior required by your SIP infrastructure:

REFER — hand the call to the destination (default) sends a SIP REFER.
Dial — bridge a new outbound leg to the destination keeps Vapi in the call path while dialing.
Bye — hang up the Vapi leg only; no transfer is placed and the destination is not dialed is for SIP systems that complete the transfer independently.

If you select Dial, set Dial Timeout (seconds) from 1 to 600 seconds. The default is 60 seconds.

Publish the tool

Select Publish in the tool header. Review the changes, then select Next. Enter a Version Name and, optionally, a Description. Select Publish to make the tool version current.

Choose a SIP verb

The SIP verb affects only calls running on SIP telephony. Other telephony providers ignore this setting.

Value	Behavior	Use it when
`refer`	Sends a SIP REFER to hand the call to the destination	The connected PBX or provider supports REFER
`dial`	Dials and bridges a new outbound call leg	The SIP system cannot complete a REFER or Vapi must bridge the destination
`bye`	Ends the Vapi call leg without dialing the configured destination	The SIP system completes the transfer through its own signaling

Do not use bye when you expect Vapi to dial the destination. It intentionally places no destination call.

Call state after a SIP transfer

The refer and dial verbs leave the call in different states once the transfer starts.

refer hands the call off with a SIP REFER. Vapi leaves the call when the transfer starts, and the destination continues the call without Vapi.
dial bridges a new outbound leg to the destination. The SIP layer keeps the caller and destination connected until either side hangs up, but the assistant leaves the call when the transfer starts. You can’t unbridge a connected dial transfer and return the caller to the original assistant.

With either verb, Vapi’s recording and transcript stop when the transfer starts. To capture the rest of the call, record it in a system that stays in the media path, such as your SIP provider or PBX.

If a refer or dial transfer is busy, unanswered, or fails, see If the transfer fails.

Verify the transfer

Place a test call and trigger the transfer. Confirm that the destination rings and that the customer and destination can hear each other.

For blind-transfer-add-summary-to-sip-header, inspect the SIP signaling at the receiving private branch exchange (PBX) or provider. Confirm that the destination INVITE or REFER flow contains X-Transfer-Summary and that the value contains the expected summary.

An assistant-forwarded-call ended reason confirms that Vapi initiated the transfer. It does not confirm that the downstream PBX or provider completed it.

Troubleshooting

For provider diagnostics and packet capture guidance, follow Troubleshoot call transfers.

Symptom	Likely cause	Resolution
The original call ends, but the destination never rings	The SIP system rejected or did not receive the REFER	Open the affected call in Calls under Logs and check its Logs tab, then inspect the provider or PBX SIP logs and packet capture. Confirm required SIP signaling addresses are allowed.
The call ends without dialing a destination	`sipVerb` is set to `bye`	Use `refer` or `dial` when Vapi should place or initiate the transfer.
A dial transfer stops ringing too soon	`dialTimeout` is too short	Increase `dialTimeout` within the 1–600 second range.
The caller doesn’t return to the assistant after a failed transfer	The destination has no `fallbackPlan`, or the call uses Twilio, Telnyx, or Vonage	On Vapi numbers and bring-your-own SIP trunks, add a `fallbackPlan` to the destination’s `transferPlan`. See If the transfer fails.
The recording or transcript stops at the transfer	Vapi’s recording and transcript end when the transfer starts for both `refer` and `dial`	Record the full call in your SIP provider or PBX.
The summary header is missing	The standard blind mode is selected or summary generation is disabled	Use `blind-transfer-add-summary-to-sip-header` and enable `summaryPlan`. Inspect the receiving SIP system rather than the audio transcript.
The summary contains the wrong information	The summary prompt is broad or uses the wrong template variable	Give `summaryPlan.messages` a focused instruction and include `{{transcript}}` in the user message.
Vapi shows `assistant-forwarded-call`, but the transfer failed	Vapi initiated the downstream transfer but did not observe its final result	Use the telephony provider’s call detail record and SIP signaling as the source of truth for downstream completion.

API reference

The Update Tool API reference documents the transfer destination and transfer-plan fields accepted when updating a reusable tool.

Transfer Call tool

Create a transfer call tool and configure shared behavior.

Warm transfer

Introduce the caller with a message, summary, or TwiML.

Assistant-based warm transfer

Let a transfer assistant speak with the destination.

Troubleshoot call transfers

Diagnose incomplete transfers and SIP routing failures.

Choose a blind transfer mode

What the caller and destination hear

If the transfer fails