SIP REFER — Inbound Calls

Transfer an inbound call from your SIP endpoint to a human agent or any phone number using SIP REFER

A caller dials your Plivo number, Plivo routes it to your AI agent or PBX, and your endpoint sends REFER to hand off to a human agent. Plivo bridges the caller to the transfer target and disconnects your endpoint. The caller never re-dials.

Use this guide when a caller dialled your Plivo number and your endpoint answered, and you now want to transfer the call to another destination. For outbound call transfers, see SIP REFER — Outbound Calls.

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Prerequisites

Requirement	Description
Inbound SIP trunk	An inbound Plivo SIP trunk with your SIP endpoint configured as the termination URI.
Plivo phone number	A Plivo number linked to your inbound trunk so calls route to your endpoint.
REFER-capable endpoint	Your SIP endpoint (PBX, softphone, AI agent) must support sending SIP REFER requests.
Transfer destination	A Plivo number or reachable E.164 number to transfer the caller to.

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How it works

Step	What happens
(1) Call established	The customer dials your Plivo number. Plivo routes the call to your SIP endpoint. Your endpoint 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.

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Step-by-step guide

Step 1: Set up your inbound trunk

1. Go to cx.plivo.com/sip-trunking → Inbound Trunks
2. Create or select an inbound trunk and set your SIP endpoint as the Primary URI
3. Assign a Plivo number to the trunk — go to the trunk → Phone Numbers → link a number

Step 2: Answer the inbound call

When a customer dials your Plivo number, Plivo sends an INVITE to your SIP endpoint URI. Your endpoint responds 200 OK to answer.

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:[e-mail]>

Use your SIP trunk domain in the Refer-To header (ending in .zt.plivo.com). Transfers to other domains or arbitrary SIP URIs are blocked.

Example REFER message:

REFER sip:[e-mail] SIP/2.0
Via: SIP/2.0/UDP 192.168.1.100:5060;branch=z9hG4bK-524287-1
From: <sip:[e-mail]>;tag=12345
To: <sip:[e-mail]>;tag=67890
Call-ID: 1-12345@192.168.1.100
CSeq: 1 REFER
Contact: <sip:endpoint@192.168.1.100:5060>
Refer-To: <sip:[e-mail]>
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. The original caller’s number is preserved as the caller ID presented to the transfer target.

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What callers experience

• Caller ID preserved: The transfer target sees the original caller’s number, not your endpoint’s SIP URI.
• 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.

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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

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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:[e-mail].
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.
Transfer target sees wrong caller ID	For inbound transfers, the original caller’s number is always passed as caller ID — this is expected behaviour.

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Related

• SIP REFER Overview — Comparison, FAQ, and restrictions
• SIP REFER — Outbound Calls — Transfer outbound calls
• SIP Trunk Debug Logs — How to read SIP traces
• SIP Trunk Hangup Codes — Full list of hangup causes