We just started to use Retall and have some questions
We are experiencing an echo in the first 2-5 seconds of the AI receptionist’s talk
We are experiencing a call transfer; the transfer tone (“During Transfer Call” music/ringback) is audible to the customer, but not audible to the AI agent or in the call recording.
From the AI agent’s perspective, there is silence, so the agent incorrectly assumes the customer is still present and starts speaking (e.g., “Are you there?”, “Hello?”, etc.).
As a result:
The customer hears only the transfer tone and does not hear the AI agent
The recipient of the transferred call experiences no issues
The AI agent and call recordings do not capture the transfer audio
The issue is only visible to engineers reviewing call recordings, not to the customer or the call recipient
This creates misleading call recordings and causes the AI agent to speak during the transfer phase when it should remain silent.
Both of these issues are very common audio-path behaviors when deploying a new voice receptionist, and they can be easily explained and addressed by fine-tuning your configuration. Here is a breakdown of what is happening under the hood and how to fix it:
1. Echo in the First 2–5 Seconds
An echo that appears only at the very start of a call and then completely disappears is the classic signature of Acoustic Echo Cancellation (AEC) settling in. When the call connects and your AI agent begins its greeting, the agent’s own voice loops back through the caller’s speakerphone or carrier network. The echo cancellation algorithm requires 2 to 4 seconds to learn this specific audio path and neutralize it.
The Fix: If you are using custom telephony (like a Twilio or Telnyx SIP trunk), verify that echo cancellation is explicitly enabled on your trunk settings. A great way to isolate this is to buy a quick Retell-managed phone number to test. If the echo disappears on the Retell number, the issue is coming from your external carrier’s transcoding.
Agent Mitigation: Inside your agent settings, temporarily raise the Interruption Sensitivity (to around 0.8 or 0.9). While this won’t eliminate the caller’s acoustic echo loop, it stops the AI agent from mistakenly interrupting itself or tripping over its own echoed greeting during those opening seconds.
2. Transfer Tone Issues & Misleading Agent Speech
This behavior occurs because of how a Warm Transfer handles separate audio legs. During a warm transfer, Retell places the caller on hold, meaning the ringback/transfer tone is generated strictly on the caller’s side of the line. Because this tone is not fed into the AI agent’s audio input channel, the agent experiences complete silence. Believing the caller is still waiting in silence, the agent’s default conversational re-prompt triggers (“Hello?”, “Are you there?”).
The good news is that the customer only hears the transfer tone and never actually hears the agent’s confused prompts. However, it creates misleading call recordings and wastes tokens. Here is how to fix it cleanly:
Switch to a Cold Transfer (Recommended for simple routing): If your receptionist just needs to hand the call off without briefing a human recipient first, switch your call transfer node to a Cold Transfer. In a cold transfer, the AI agent disconnects the exact millisecond the transfer is initiated. This completely cuts the agent out of the loop and eliminates the “talking into silence” recording artifact.
Use Native Transfer Tools (If keeping Warm Transfer): If you absolutely need a warm transfer (e.g., to pass a briefing message to the human recipient), make sure you are using Retell’s native Transfer Call function tool rather than a generic prompt instruction or custom code. The native tool explicitly manages the caller’s hold state and directs the agent’s listening focus toward the transfer target to detect a human voice instead of re-prompting the customer.
Note for Reviewers: When auditing performance, look at the transfer_call tool invocation marker in your call transcripts rather than the raw mixed audio leg to accurately evaluate when the receptionist successfully completed its task.
Let us know if making these routing adjustments clears up your call logs!