Audible tone during cold transfer sounds like a dropped call — began within the last week

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1

Audible tone during cold transfer sounds like a dropped call — began within the last week

We use transfer_call with transfer_option.type: "cold_transfer" and cold_transfer_mode: "sip_invite" across several of our voice agents. Starting I believe within the last week, callers now hear an audible tone/open-line sound during the transfer handoff that was not present before. To callers, it sounds indistinguishable from being hung up on — several have asked “did you just hang up on me?” mid-transfer.

Nothing on our side changed in how we configure these transfer_call tools. This started happening on calls without any corresponding change to our agent or LLM configuration, which suggests something changed on Retell’s platform side (possibly in SIP INVITE transfer signaling/audio bridging).

Affected agents (all using cold_transfer / sip_invite):

  • Houlihan Lawrence — agent_95b06b9c0765f6770e5465949d

Sample call IDs demonstrating the issue:

  1. call_6209634ee3e8680373408d84fe8
  2. call_22f9f3e5879ac37092fc56cecb4

Questions:

  1. Was there a recent platform-side change to cold_transfer / SIP INVITE handling that could introduce this tone?
  2. Is there any way to suppress or control this audio cue for cold_transfer? (Your docs show on_hold_music / bridge-audio-cue controls only exist for warm_transfer — cold_transfer appears to have none.)
  3. If this is unintended/a regression, can it be fixed or rolled back on your end?

As a workaround, we’ve added a spoken warning before the transfer so callers know to expect the tone, but we’d like this resolved properly at the platform level.

2

Hello @supernova77 Checking this with the team and will get back to you as soon as I have an update.

Thank You

3

Hello @supernova77 Thanks for the detailed report and the sample calls.

  • You’re right that something changed on our side. We recently moved outbound and transfer legs onto a new SIP media path, and on that path a ringback/dial tone is played to the caller during the ring phase of a cold sip_invite transfer. Both sample calls you sent were handled on the new path, which is why callers now hear a tone during the handoff.
  • We’ve surfaced this to engineering to review whether the default tone should be kept for cold_transfer on the new path and whether to expose a suppression option at parity with warm_transfer.

I don’t have a timeline yet — I’ll follow up here as soon as there’s an update.

Thank You

4

Hey @supernova77 To address your remaining question, there is currently no customer-facing setting to disable or customize this tone for cold_transfer with sip_invite.

A possible workaround to test is to use Warm Transfer with On-hold Music = Ringtone, and disable:

  • Set “Is there any internal queue or hold system before an agent answers?” to No
  • Turn off Navigate IVR.
  • Turn off Whisper Debrief Message and Three-Way Debrief Message.
  • Turn off Three-Way Ring Tone.

Documentation: Transfer call tool (single & multi-prompt agents) - Retell AI

This should play the selected ringtone while dialing, then connect the caller when the destination answers, without an AI handoff message. With human detection disabled, the caller will also be connected if an IVR or voicemail answers.

Could you try this on a test call and let us know whether it works for your use case?

Thank You

5

Is there no fix for this. we transfer to a ringing system and it’s caused a massive issue with clients!

6

Hey @efondren Could you please share the relevant Call IDs? This will help us investigate the issue further.

Thank You

7

call_47b660c6638b3c27105270e035a - this is just one call, there have been many over the past few days that ended this way

8

Hey @efondren At the moment, the dialtone before the transfer target actually starts ringing is by design. However, I understand how this can be confusing and causing issues, I’ve raised this internally and added this as a feature request, but I don’t have a fixed timeline for implementation of a fixing change at the moment.
For now previously mentioned solution in thread you can follow that to give play a ringing sound immediately:
A possible workaround to test is to use Warm Transfer with On-hold Music = Ringtone, and disable:

  • Set “Is there any internal queue or hold system before an agent answers?” to No
  • Turn off Navigate IVR.
  • Turn off Whisper Debrief Message and Three-Way Debrief Message.
  • Turn off Three-Way Ring Tone.

Documentation: Transfer call tool (single & multi-prompt agents) - Retell AI

Thank You