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Add card-on-file payments to an OpenAI Realtime agent by call transfer. This works like a hosted web checkout, over the phone: when the order is confirmed, your bridge creates a Checkout Session and transfers the live call to a Ringup payment line, the caller pays there, and the call optionally returns to your agent. Ringup recognizes the caller, charges their saved card, settles it against the order, and texts a receipt. Your server reconciles from a webhook. If the model is new to you, read Call transfer first. With the Realtime API you bring the transport (WebRTC, a WebSocket, or the SIP connector) and the telephony under it, so your bridge owns the call leg. The transfer happens on that bridge, and setup is code there. This page is end to end: setup, the call, the return, and what happens after payment.

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How the pieces map to OpenAI Realtime

Ringup pieceOn your Realtime bridge
create_checkoutAn HTTP POST your bridge makes when the order is confirmed
The transfer targetThe transfer_to SIP URI the Checkout Session returns
The transferA <Dial><Sip> (or SIP REFER) on your telephony to the Ringup SIP URI
The resultA checkout.completed webhook to your server

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Prerequisites

  • A running OpenAI Realtime agent reachable over the phone (Twilio media streams, the SIP connector, or your own SIP trunk).
  • The telephony account that owns the inbound call leg.
  • A Ringup account and API key. Test mode works out of the box on a shared sandbox. See Testing.

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Step 1: Create a Checkout Session when the order is confirmed

Give the model a tool (a Realtime function) that signals the order is ready to pay, and handle that tool call in your bridge by calling Ringup. There is no platform tool to register with Ringup; this is a plain HTTP call from your server with your Ringup API key. 
POST https://api.ringup.dev/v1/checkouts
Authorization: Bearer <YOUR_RINGUP_KEY>
Content-Type: application/json

{
  "amount_cents": 2500,
  "line_items": [{ "name": "Large pepperoni", "quantity": 1, "amount_cents": 2500 }], // optional
  "order_id": "sq_abc",                              // optional: settle against an existing order
  "return_to": "sip:your-bridge@your-telephony",     // optional: transfer the caller back after payment
  "success_message": "You are all set, your order will be ready shortly."  // optional (if no return_to)
}
Response (the Checkout Session): 
{
  "id": "cs_x9f2",                                   // Checkout Session id; the correlation token
  "payment_required": "required",                    // "required" | "optional" | "none"
  "transfer_to": "sip:cs_x9f2@transfer.ringup.dev"   // where to transfer; embeds the id
}
If payment_required is none, do not transfer: finish the call normally. Otherwise transfer.

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Step 2: Transfer on your bridge

Have the agent speak one short bridge line (“One moment, connecting you to secure payment”), then transfer the caller on your telephony. On Twilio, redirect the call to TwiML that dials the Ringup SIP URI, carrying the Checkout Session id: 
<Response>
  <Dial>
    <Sip>sip:cs_x9f2@transfer.ringup.dev?X-Session-Id=cs_x9f2</Sip>
  </Dial>
</Response>
If your call arrives over the SIP connector, a SIP REFER to the same URI works too. Because your bridge controls the telephony, the header path is reliable: the URI parameter reaches the Ringup line, which matches it to the Checkout Session.

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Step 3: The return, and the result (the webhook)

Ringup answers the transferred call, recognizes the Caller, charges the saved Card, and texts a receipt. A first-time caller with no saved card is texted a secure pay link instead. Then the call ends one of three ways, from what you passed in Step 1:
  • return_to set: Ringup transfers the caller back to your bridge with the outcome in SIP headers (X-Payment-Status, X-Confirmation), so your agent resumes and closes the call.
  • success_message / failure_message set (no return_to): Ringup reads your line and ends.
  • Neither: Ringup reads a short default and ends. The receipt is the proof.
A return_to that cannot connect falls back to reading your success_message (or the default), so a failed return never strands the caller. Either way, your server reconciles from a webhook: 
{
  "id": "evt_9f2",
  "type": "checkout.completed",         // or "checkout.failed"
  "data": {
    "checkout_id": "cs_x9f2",
    "payment_id": "pay_abc",
    "amount_cents": 2500,
    "order_id": "sq_abc",               // echoed back
    "confirmation": "2PJJZY",
    "caller": "+14155551234",
    "card": { "brand": "VISA", "last_4": "5858" }
  }
}
Match on the order_id you passed, store the payment_id, and flip your order to paid. See Webhooks for the full catalog, statuses, signature verification, and idempotency.

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Validation status

Because your bridge owns the telephony, the transfer is standard <Dial><Sip> (or SIP REFER) on your provider. Ringup’s hosted transfer endpoint is rolling out; confirm access in your Ringup dashboard before relying on this in production. The recognition, charge, receipt, and webhook behavior is the same payment path Ringup runs everywhere.