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docs: explain Telnyx warm-transfer caller ID preservation #7310

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54 changes: 53 additions & 1 deletion examples/warm-transfer/README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -22,7 +22,7 @@ The `WarmTransferTask` from `livekit.agents.beta.workflows` simplifies the warm

```python
result = await WarmTransferTask(
target_phone_number=SUPERVISOR_PHONE_NUMBER,
sip_call_to=SUPERVISOR_PHONE_NUMBER,
sip_trunk_id=SIP_TRUNK_ID,
chat_ctx=self.chat_ctx, # Provides conversation history to the supervisor
)
Expand All @@ -47,3 +47,55 @@ result = await WarmTransferTask(
```python
python warm_transfer.py dev
```

## Preserve the inbound caller's number with Telnyx

A warm transfer creates a new outbound call to the supervisor. To let the
supervisor identify the customer, an application can preserve the customer's
caller ID instead of presenting its business number. The existing `sip_number`
and `sip_headers` options support this with a Telnyx SIP trunk.

For an inbound call from customer A to your Telnyx number B, followed by a
transfer to supervisor C, [Telnyx requires](https://support.telnyx.com/en/articles/13117410-how-external-call-transfers-work):

- The original A-to-B call must still be active when dialing C.
- The outbound call must present A as the caller and include a SIP `Diversion`
header identifying B.

In `SIPSupportAgent._start_transfer`, use the following options when creating
the task. `inbound_customer_number` and `inbound_telnyx_number` are application
variables: retrieve the original caller and called number from trusted,
server-side context for this specific inbound call. Normalize both to E.164
before constructing the header. `SIP_TRUNK_ID` must select your configured
Telnyx outbound trunk.

```python
return await WarmTransferTask(
sip_call_to=SUPERVISOR_PHONE_NUMBER,
sip_trunk_id=SIP_TRUNK_ID,
sip_number=inbound_customer_number,
sip_headers={
"Diversion": f"<sip:{inbound_telnyx_number}@sip.telnyx.com>;reason=unconditional",
},
chat_ctx=self.chat_ctx,
extra_instructions=SUMMARY_INSTRUCTIONS,
)
```

This uses the existing SIP warm-transfer flow; it does not require a CallToken
or a Telnyx-specific SDK flag. Keep the customer's original call connected
during consultation. Applications that want their business caller ID can keep
the existing example configuration.

### Verify and troubleshoot

Call B from phone A, request a transfer to phone C, then check the number shown
on C and complete the consultation and transfer. Caller-ID presentation also
depends on the receiving network. This snippet illustrates configuration; it
does not by itself verify carrier acceptance.

For `403 Unverified origination number D51`, check that the A-to-B call is still
active, the outbound caller number matches A, and `Diversion` identifies the
actual Telnyx number B that received the call. Inspect the outbound SIP INVITE
to confirm that the header reaches Telnyx. An arbitrary customer number or a
header copied from a different call does not establish a valid transfer.