What is call transfer in my SIP PBX?

Dropped handoffs waste minutes and lose deals. You need transfers that just work, inside your PBX and across trunks.

Call transfer moves an active call to another endpoint. It works in two main modes—blind and attended—and most systems use SIP REFER to complete the handoff.

Let me break it down in plain steps. I will show how blind and attended transfers differ, how REFER works, what your phones need, how to turn features on, and how to fix failures fast.

How do I perform blind vs attended transfer on my system?

Different jobs need different handoffs. One is instant. One is careful.

Blind transfer sends the caller to the target right away. Attended transfer first consults the target, then completes the move when ready.

The two transfer styles at a glance

Here is a simple table that shows the difference.

Why these modes matter

Blind transfer shines when speed beats context. A receptionist sends a caller to billing. A queue agent moves a wrong-queue call to sales. The key is low friction. If the target does not answer, the PBX should apply no-answer rules. This may mean ring group, voicemail, or fallback to the operator.

Attended transfer is safer for high-stakes calls. You place the caller on hold. You call the target. You confirm that the target is free and ready. Then you complete the handoff. In SIP, many phones signal this with a REFER that carries a Replaces header ¹. That allows the two call legs to join cleanly.

Real tips that save time

• Put Transfer and Attended Transfer on separate softkeys. Less confusion, fewer misses.
• Train people to announce the caller’s name and need in the consult leg. This keeps handoffs warm and polite.
• For call centers, map blind transfer to common destinations using BLF/Speed Dial keys.
• For executives, force attended transfer to avoid sending callers to voicemail by accident.

Do my IP phones support transfer using SIP REFER?

Most IP phones do. Some trunks do not. That gap causes pain.

Check the phone’s SIP settings for SIP REFER ² support and Replaces. Also check if your SIP trunk allows REFER, or the PBX must stay in the media path.

What to verify on the endpoint

Open the phone’s web UI or provisioning template. Look for:

• Enable REFER or Call Transfer via REFER
• REFER-To handling and Replaces header support
• NOTIFY on REFER subscriptions (for result updates)
• SIP 302 redirection ³ setting, if your device can deflect instead of REFER
• Hold music (MOH) source during attended consult

Many desk phones from leading vendors ship with REFER enabled by default. Softphones often allow both REFER and “transfer via PBX feature code.” For reliability, keep firmware current. Vendors sometimes fix REFER edge cases in maintenance releases.

What to verify on the PBX and SBC

Even if your phone supports REFER, your trunk may block it on off-net calls. Two outcomes are common:

1. End-to-end REFER: The PBX passes REFER to the trunk. The far side accepts it. The PBX drops out. Clean and efficient.
2. Hairpinned transfer: The trunk rejects REFER. The PBX converts the transfer into a new INVITE and bridges media. You use two trunk channels, but the call still completes.

Some Session Border Controllers (SBCs) ⁴ can terminate REFER locally. They create the new INVITE themselves, even if the phone cannot. This is useful when you mix devices with uneven SIP support.

A simple capability checklist

If any box is “No,” the system can still transfer, but it may keep media anchored on the PBX, or require feature codes.

How do I enable transfer features on my IP PBX?

Features live in three places: phone keys, PBX call control, and trunk policy.

Turn on REFER at the phone, allow it on the PBX or SBC, and configure fallbacks. Add feature codes for attended and blind as a backup path.

Step 1: Decide your default transfer path

Pick one:

• REFER first, fall back to PBX-mediated transfer
• Always PBX-mediated transfer for maximum control
• Redirection (302) for internal moves, REFER for external, as allowed

This choice sets how your PBX, phones, and trunks will interact. In many offices, REFER for internal calls gives a clean handoff. For outbound transfers to mobiles or carriers, some admins prefer PBX-mediated moves to keep recording and policy controls.

Step 2: Configure device keys and templates

• Map Transfer and Attended Transfer softkeys.
• Add BLF/Speed Dial for common targets.
• Enable Replaces and REFER in the SIP profile.
• Push these via provisioning so every phone matches policy.

Step 3: Tune PBX call control

• Enable REFER processing and NOTIFY result handling.
• Allow or block REFER on trunks per route.
• Set attended transfer timeout (how long you can consult).
• Pick music on hold for the parked caller.
• Define fallback rules: if transfer fails, resume the original call, or send to operator.
• Enable Release Link Trunking (RLT) if your trunk supports it. RLT lets blind transfers clear the PBX out of the path after the far ends connect.

Step 4: Add feature codes and user rights

Publish simple codes like:

• *1 Blind transfer (while on a call, dial *1 + extension)
• *2 Attended transfer (park caller, call target, then *2 to complete)

Assign class of service so only trusted roles can transfer to external numbers.

Step 5: Log, monitor, and train

Turn on SIP trace or CDR flags for transfers. Train staff with short call-flows. Keep a one-page cheat sheet near the phones. When people know which key to press, failure rates drop fast.

Why does my call transfer fail or drop calls?

Most failures come from policy blocks, header issues, or early media problems.

Check trunk policy for REFER/302, confirm Replaces support, and watch Session Description Protocol (SDP) ⁵/early media. If still broken, force PBX-mediated transfer and test again.

Common failure patterns and quick fixes

A short diagnostic workflow

1. Reproduce with a known pair: internal A → internal B. If that works, try A → mobile.
2. Capture SIP: Look for REFER from phone to PBX, 202 Accepted, and NOTIFY with success codes.
3. Check headers: Refer-To, Referred-By, and Replaces must be correct and routable.
4. Watch the SDP: Confirm that final OK messages advertise reachable media addresses and a common codec.
5. Toggle policies: If the trunk blocks REFER, test PBX-mediated transfer. If that works, you found the policy gap.
6. Retry with 3xx: Some carriers accept a 302 Moved Temporarily redirection even if they refuse REFER.
7. Security check: Limit REFER to trusted devices. Do not allow unauthenticated third-party targets on open SIP.

Notes on reliability and security

Transfers touch many legs. Each hop adds risk. Keep endpoints updated. Lock REFER behind authentication. Monitor for patterns that hint at abuse, like repeated transfers to premium numbers. This is simple, but it closes a big door to PBX toll fraud ⁷. For external calls, enable call recording or compliance as needed. If you must keep the PBX in the media path, size trunks for hairpinned calls during peak hours.

Conclusion

Transfer should be simple: choose blind or attended, enable REFER with safe policies, and keep a clean fallback so calls never drop.

Footnotes