https://retellai.com/?utm_source=discord&utm_medium=seo logo
Powered by
Here's the updated message:Hi, we're having an iss...
# ❓・ai-evy-support
b
@AI Evy Here's the updated message: Hi, we're having an issue with call transfers not completing to voicemail on no-answer. Setup: Inbound calls come in via Twilio imported number (SIP trunk) Transfer type: Cold Transfer SIP Transfer Method: SIP REFER Transfer Ring Duration: 30 seconds PSTN Transfer: enabled on Twilio trunk Caller ID for Transfer Target: Set as Transferor Twilio trunk: Call Transfer (SIP REFER) enabled Issue: When the agent transfers a call to a team member and they don't answer, instead of the call rolling to the team member's voicemail, Retell returns the call back to the agent with the status: "Transfer call cannot be completed, the other side did not pick up." We have tested with one number (consumer cell phone) and the transfer DID successfully roll to voicemail. But with other numbers — including at least one 416 area code number which may be a business line — the transfer fails and bounces back to the agent instead of hitting voicemail. Questions: When a cold transfer via SIP REFER times out on no-answer, should the call roll to the destination's voicemail automatically, or does Retell reclaim the call after the ring duration? Is there a configuration on your end or Twilio's end that would prevent the call from rolling to voicemail on no-answer? Would increasing the Transfer Ring Duration help in cases where the carrier takes longer to roll to voicemail?
a
Thanks for tagging me @bohmez. I'll look through relevant sources and get back to you shortly.
### Answer When using cold transfer with SIP REFER, the behavior you're experiencing is expected based on how Retell handles the transfer ring duration. [(1) ]() [(2) ]() Ring Duration Behavior: The Transfer Ring Duration setting controls how long the destination should ring before Retell considers the transfer unsuccessful. [(1) ]() When set to 30 seconds, if the transfer target doesn't answer within that timeframe, Retell will mark the transfer as failed and return control to the agent. [(1) ]() [(3) ]() [(4) ]() SIP REFER and Voicemail: With SIP REFER cold transfers, the behavior depends on your telephony provider's configuration. [(5) ]() For Twilio specifically, the documentation notes that different telephony providers may or may not honor custom SIP headers in REFER requests, and that caller ID override support varies by provider. [(2) ]() [(5) ]() Your Questions: 1. Does the call roll to voicemail automatically? When the ring duration expires without an answer, Retell reclaims the call and marks the transfer as failed. [(1) ]() The call does not automatically continue to voicemail after the configured ring duration. 2. Configuration preventing voicemail rollover: The Transfer Ring Duration setting on Retell's side determines
when the transfer times out. If the carrier takes longer than your configured duration to roll to voicemail, Retell will reclaim the call first. 3. Would increasing Transfer Ring Duration help? Yes, increasing the Transfer Ring Duration beyond 30 seconds could help if the carrier needs more time to route to voicemail. The valid range is 5,000 to 300,000 milliseconds (5 seconds to 5 minutes) . [(1) ]() [(3) ]() [(4) ]() Note on Deprecation: Be aware that as of January 23, 2026, the behavior of caller ID settings in cold transfers has changed. [(6) ]() The 
show_transferee_as_caller
parameter no longer controls transfer mode selection - you should now use the 
cold_transfer_mode
parameter to explicitly choose between 
sip_refer
and 
sip_invite
.
Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.
b
@AI Evy Following the January 23 2026 deprecation of show_transferee_as_caller, what is the correct configuration to ensure the original caller's number is shown on caller ID when transferring via SIP REFER cold transfer? Where do we set cold_transfer_mode?
a
Thanks for the follow-up, @bohmez. I'll get back to you soon.
### Answer --- Following the January 23, 2026 deprecation, here's the correct configuration: [(1) ]() Where to set `cold_transfer_mode`: The 
cold_transfer_mode
parameter is set in the cold transfer configuration when creating or updating your agent. The affected APIs include: - Create Retell LLM - Update Retell LLM - Create Conversation Flow - Update Conversation Flow - Create Conversation Flow Component - Update Conversation Flow Component Correct Configuration for SIP REFER with Original Caller ID: Set 
cold_transfer_mode
to 
sip_refer
to explicitly choose the SIP REFER transfer method. After the deprecation, 
show_transferee_as_caller
only controls caller ID display and only takes effect when 
cold_transfer_mode
is set to 
sip_invite
. Since you're using SIP REFER, the 
show_transferee_as_caller
parameter will not affect your transfer. Important Note on Caller ID with SIP REFER: For cold transfer using SIP REFER, caller ID behavior depends on your telephony provider's support and configuration. [(2) ]() With Twilio, caller ID override is supported when using SIP REFER via cold transfer. [(3) ]() However, you need to ensure your Telnyx/Twilio trunk has the following enabled: [(4) ]() - Enable SIP REFER - Enable transfer to PSTN - Set caller ID as transferee (From-header passthrough) Summary: - Set `cold_transfer_mode`: 
sip_refer
[(1) ]() - Ensure your telephony provider supports caller ID override for SIP REFER [(2) ](
onversation-flow/call-transfer-node) [(3) ]() - Verify trunk configuration allows From-header passthrough [(4) ]()
​ To learn more, see the sources I considered: [[1] Deprecating on 01/23/2026](https://docs.retellai.com/deprecation-notice/2026/01-23_cold_transfer_mode_selection) [[2] Call Transfer Node](https://docs.retellai.com/build/conversation-flow/call-transfer-node) [[3] Transfer call](https://docs.retellai.com/build/single-multi-prompt/transfer-call) [[4] the original caller or callee to be displayed when COLD transferring a call](https://discord.com/channels/1186424376300679209/1394983296521343118) ​
Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.
Thanks for the feedback, glad I was helpful.
PreviousNext
Powered by