Connect your phone system

Natural’s agent takes the card on its own leg of the call, so when your phone system is set up as described here, the caller speaks their card details only to Natural. Card details never reach your agent, recordings, transcripts, webhooks, or servers, which keeps you out of scope of hearing card data.

How the handoff works

Your agent hands the caller off with a blind SIP transfer and leaves the call, and your carrier, not your software, connects the caller to Natural.

Your agent creates a voice session and gets its dialUri.
Your agent asks your carrier to transfer the caller to dialUri with a blind SIP REFER, then leaves the call.
Your carrier connects the caller to Natural over an encrypted leg, where Natural’s agent takes the card and charges it.
Natural asks your carrier to transfer the caller back to your returnTarget, or hangs up if you didn’t set one.

Requirements

Every setup must meet all of these, and Natural checks them with you before you go live.

Requirement	Why
Your number is on a SIP trunk, not a programmable voice app	A voice app (Twilio TwiML, Telnyx Call Control or TeXML) keeps control of the caller’s leg during card entry and can record the call.
Hand off with a blind SIP REFER	A bridge, conference, warm transfer, or new call you place to `dialUri` keeps your agent on the line while the card is read.
TLS signaling and SRTP media on the trunk	Natural accepts calls only over TLS on port 5061 and refuses calls without SRTP audio.
Recording, transcription, and audio streaming off on your carrier	Carrier recording follows the caller into card entry.
`dialUri` comes only from Natural’s API	Take it from the create response for that session, and never build it from the conversation, so nobody can redirect the caller. On Telnyx, swap only the host, as the Telnyx guide shows.

Natural supports Twilio and Telnyx today, so if you use a different carrier, contact hi@natural.com.

Choose your carrier

Twilio

Elastic SIP Trunking and LiveKit

Telnyx

SIP connection and LiveKit

Choose your agent platform

The carrier guides use LiveKit, but any platform works if it sends a SIP REFER and leaves the call. The call must reach the platform over your own Twilio or Telnyx SIP trunk, because numbers bought inside these platforms sit on the platform’s carrier account, where you can’t control recording, encryption, or transfers. Several platforms bridge by default, so set the transfer mode explicitly.

Platform	Use	Avoid
LiveKit	`TransferSIPParticipant` (cold transfer)	`CreateSIPParticipant` to `dialUri`, warm transfer
Vapi	`sipVerb: "refer"` (the default)	`sipVerb: "dial"`
Retell	SIP Transfer Method: SIP REFER	SIP INVITE (the default)
ElevenLabs	`transfer_type: "sip_refer"`	`conference` (the default)
Bland	”Hand off via SIP REFER”	New INVITE, `stream_post_transfer_transcript`
Pipecat on Daily	`sipRefer`	`sipCallTransfer`

Get the caller back

Set returnTarget to a SIP address your agent platform answers, such as your LiveKit SIP host, with a fixed user that marks returning calls:

sip:pay-return@<your-sip-host>;transport=tls

When the payment is done, Natural asks your carrier to transfer the caller to this address. The caller reaches your agent as a brand-new inbound call, so your agent needs two checks:

Is this a returning caller? A returning call is addressed to pay-return, while a new call is addressed to your phone number.
Which conversation is it? Look it up by the caller’s phone number, and accept the match only if you handed that caller to Natural within the last 30 minutes, the longest a payment call can run, since caller ID alone can be faked.

The carrier guides show both checks in LiveKit.

Go live

Before your first live call, Natural reviews your setup with you, and together you place test calls in Sandbox to confirm card details never reach your systems. Check each test call for these:

Your agent’s logs show no speech-to-text after the transfer.
Your agent left the call within a second of the transfer.
Your carrier has no recording or transcript of the call.
Your webhooks received no key presses.
The caller heard Natural’s agent, and after they read the Sandbox test card, the session’s payment.outcome is paid.

Contact hi@natural.com to schedule the review.

Overview

Common use cases

Guides