> ## Documentation Index > Fetch the complete documentation index at: https://plivo.com/docs/llms.txt > Use this file to discover all available pages before exploring further. # 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](/sip-trunking/concepts/sip-refer-outbound). *** ## Prerequisites | Requirement | Description | | -------------------------- | ------------------------------------------------------------------------------------------------------------------------- | | **Inbound SIP trunk** | An [inbound Plivo SIP trunk](https://cx.plivo.com/sip-trunking) 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. | *** ## How it works ```mermaid theme={null} sequenceDiagram participant C as Customer participant Z as Plivo (SIP Trunk) participant E as Your Endpoint participant T as Transfer Target C->>Z: (1) INVITE Z->>E: (1) INVITE E-->>Z: (1) 200 OK Z-->>C: (1) 200 OK Note over C,E: Call established — customer talking to endpoint E->>Z: (2) REFER (Refer-To: Transfer Target) Z-->>E: (3) 202 Accepted Z-->>E: (3) NOTIFY 100 Trying Z->>T: (4) INVITE T-->>Z: (4) 180 Ringing Z-->>E: (4) NOTIFY 180 T-->>Z: (5) 200 OK Z-->>E: (5) NOTIFY 200 OK Z->>E: (6) BYE Note over C,T: Customer and Transfer Target connected ``` | 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. | *** ## Step-by-step guide ### Step 1: Set up your inbound trunk 1. Go to [cx.plivo.com/sip-trunking](https://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:** ```text theme={null} Refer-To: ``` 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:** ```sip theme={null} 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: ;tag=12345 To: ;tag=67890 Call-ID: 1-12345@192.168.1.100 CSeq: 1 REFER Contact: Refer-To: 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. *** ## 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. *** ## 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](/sip-trunking/concepts/geo-permissions). | | **No audio after transfer** | Codec mismatch between legs. Contact [Plivo Support](https://support.plivo.com) 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. | *** ## Related * [SIP REFER Overview](/sip-trunking/concepts/sip-refer) — Comparison, FAQ, and restrictions * [SIP REFER — Outbound Calls](/sip-trunking/concepts/sip-refer-outbound) — Transfer outbound calls * [SIP Trunk Debug Logs](/sip-trunking/troubleshooting/zentrunk-debug-logs) — How to read SIP traces * [SIP Trunk Hangup Codes](/sip-trunking/troubleshooting/zentrunk-hangup-codes) — Full list of hangup causes