SIP REFER

Your AI agent or PBX places an outbound call through Plivo, then sends REFER mid-call to hand off to a human agent. Plivo bridges the caller to the transfer target and disconnects your endpoint. No re-dial, no dropped call.

Use this guide when your endpoint placed the outbound call and wants to transfer it to another destination. For inbound call transfers, see SIP REFER — Inbound Calls.

Prerequisites

Requirement	Description
Outbound SIP trunk	An outbound Plivo SIP trunk with your endpoint registered via IP ACL or credentials.
REFER-capable endpoint	Your SIP endpoint (PBX, softphone, AI agent) must support sending SIP REFER requests.
Phone numbers	At least two Plivo numbers — one for the customer leg, one as the transfer destination.

How it works

Step	What happens
(1) Call established	Your endpoint dials the customer via your Plivo SIP trunk. The customer answers.
(2) REFER	Your endpoint sends REFER with the transfer target’s number in the `Refer-To` header.
(3) Accepted	Plivo responds `202 Accepted` and sends `NOTIFY 100 Trying`.
(4) New leg	Plivo places a new call to the transfer target.
(5) Connected	Transfer target answers. Plivo sends `NOTIFY 200 OK`.
(6) BYE	Plivo disconnects your endpoint. Customer and transfer target continue the call.

Step-by-step guide

Step 1: Set up your outbound trunk

Go to cx.plivo.com/sip-trunking → Outbound Trunks
Click Create New Outbound Trunk, create a credentials list, and save
Copy the Termination SIP Domain — for example, XXXXXXXXXXXX.zt.plivo.com

Step 2: Place the outbound call

Your endpoint dials the customer via your SIP trunk domain:

sip:<customer-number>@XXXXXXXXXXXX.zt.plivo.com

Step 3: Send the REFER

When ready to hand off, send a REFER with the transfer target in the Refer-To header. Refer-To header format:

Refer-To: <sip:+14155551234@XXXXXXXXXXXX.zt.plivo.com>

Use your SIP trunk domain in the Refer-To header. Transfers to other domains or arbitrary SIP URIs are blocked.

Example REFER message:

REFER sip:trunk@XXXXXXXXXXXX.zt.plivo.com SIP/2.0
Via: SIP/2.0/UDP 192.168.1.100:5060;branch=z9hG4bK-524287-1
From: <sip:endpoint@pbx.example.com>;tag=12345
To: <sip:trunk@XXXXXXXXXXXX.zt.plivo.com>;tag=67890
Call-ID: 1-12345@192.168.1.100
CSeq: 1 REFER
Contact: <sip:endpoint@192.168.1.100:5060>
Refer-To: <sip:+14155551234@XXXXXXXXXXXX.zt.plivo.com>
Max-Forwards: 70
Content-Length: 0

Step 4: Handle NOTIFY messages

After 202 Accepted, Plivo sends NOTIFY messages to report transfer progress. Your endpoint must respond to each NOTIFY with 200 OK.

NOTIFY status	Meaning	What to do
`SIP/2.0 100 Trying`	Plivo is calling the transfer target	Show “Transferring…” in your UI. Keep the call active.
`SIP/2.0 180 Ringing`	Transfer target is ringing	Optionally relay ring-back tone to the caller.
`SIP/2.0 200 OK`	Caller and target are connected	You may hang up your leg (or wait for Plivo’s BYE).
`SIP/2.0 4xx` / `5xx`	Transfer failed	Caller is still on your line — retry or continue the call.

Do not send BYE before receiving NOTIFY 200 OK. Sending BYE early drops the caller before the transfer completes.

Step 5: Call continues between customer and transfer target

Plivo bridges the caller to the transfer target and sends your endpoint a BYE.

What callers experience

During the transfer: The caller hears ring-back from the transfer target (180 Ringing) until the target answers.
If the transfer fails: The caller remains on your leg. You can retry with a different number or continue the conversation.
Music on hold: To play hold music while the transfer leg is establishing, play audio from your endpoint before sending REFER.

Response codes

Code	Meaning
`202 Accepted`	REFER accepted — transfer is in progress
`400 Bad Request`	Missing or malformed `Refer-To` header
`403 Forbidden`	`Refer-To` domain is not a valid Plivo SIP trunk domain, or transfer target is a private IP

Troubleshooting

Symptom	Check
403 Forbidden	Verify the `Refer-To` domain ends in `.zt.plivo.com`. Ensure the target is not a private IP.
400 Bad Request	Verify the `Refer-To` header uses E.164 format: `sip:+14155551234@trunk.zt.plivo.com`.
202 Accepted but caller is dropped	Your endpoint sent BYE before receiving `NOTIFY 200 OK`. Wait for `NOTIFY 200 OK` first.
202 Accepted but no NOTIFY received	Ensure your endpoint listens on the same IP/port it advertised in the `Contact` header.
Transfer fails (NOTIFY 4xx/5xx)	Check that the destination is valid, reachable, and not busy. Verify account balance.
Transfer to international number returns 403	Enable that destination under Geo Permissions.
No audio after transfer	Codec mismatch between legs. Contact Plivo Support with the `Call-ID`.

SIP REFER Overview — Comparison, FAQ, and restrictions
SIP REFER — Inbound Calls — Transfer inbound calls
LiveKit Integration — Connect LiveKit AI agents via Plivo SIP trunking
SIP Trunk Debug Logs — How to read SIP traces
SIP Trunk Hangup Codes — Full list of hangup causes

Concepts

Security

API Reference

Troubleshooting

PBX Integration

SIP REFER — Outbound Calls

Prerequisites

How it works

Step-by-step guide

Step 1: Set up your outbound trunk

Step 2: Place the outbound call

Step 3: Send the REFER

Step 4: Handle NOTIFY messages

Step 5: Call continues between customer and transfer target

What callers experience

Response codes

Troubleshooting

​Prerequisites

​How it works

​Step-by-step guide

​Step 1: Set up your outbound trunk

​Step 2: Place the outbound call

​Step 3: Send the REFER

​Step 4: Handle NOTIFY messages

​Step 5: Call continues between customer and transfer target

​What callers experience

​Response codes

​Troubleshooting

​Related

Prerequisites

How it works

Step-by-step guide

Step 1: Set up your outbound trunk

Step 2: Place the outbound call

Step 3: Send the REFER

Step 4: Handle NOTIFY messages

Step 5: Call continues between customer and transfer target

What callers experience

Response codes

Troubleshooting

Related