> ## Documentation Index
> Fetch the complete documentation index at: https://docs.ringup.dev/llms.txt
> Use this file to discover all available pages before exploring further.

# Caller and merchant identity

> Both identities are resolved server-side from call context. The model never types either one.

The two identities that matter on a payment call, who is calling and which business they
called, never pass through the language model. Both arrive at Ringup on trusted headers the
platform templates in at call time.

## The caller: recognized by phone number

The caller's number rides the `X-Caller-Number` header, templated from the platform's
native caller-ID variable (on Vapi, `{{customer.number}}`). When the payment step begins, it
looks the caller up:

```json theme={null}
{
  "known": true,
  "name": "Alex",
  "card": { "brand": "VISA", "last_4": "5858", "exp_month": 7, "exp_year": 2028 }
}
```

A recognized caller is offered their saved card by name and last four, with no identity
questions. An unknown caller is offered a texted pay link instead; once they pay, they are
recognized on every call after that.

## The merchant: resolved from the dialed number

The number the caller dialed rides the `X-Ringup-Merchant` header (on Vapi,
`{{phoneNumber.number}}`). The dialed number is the near-universal per-merchant key on
multi-tenant voice platforms: one number per business, no per-agent configuration. Ringup
maps it to the merchant server-side and applies that merchant's policy and processor.

A platform that prefers explicit tenant ids can send its own merchant identifier in the same
header instead; both forms resolve server-side.

## Why headers, not tool arguments

A model-generated `phone_number` or `merchant_id` argument can be mistyped, hallucinated, or
prompt-injected by a hostile caller. A header set by the platform's template engine cannot.
The tools accept no identity arguments at all; there is nothing to spoof.

<Warning>
  Dashboard and web test calls carry no real caller ID, so the platform's caller-ID template
  resolves to nothing. For those, pin a test caller explicitly (the Vapi CLI's
  `--demo-caller` flag). Real inbound phone calls resolve both identities natively.
</Warning>