​ To learn more, see the sources I considered: [[1] LLM WebSocket](https://docs.retellai.com/api-references/llm-websocket) [[2] Call transfer node in conversation flow](https://docs.retellai.com/build/conversation-flow/call-transfer-node) [[3] Phone transfer with custom LLM](https://discord.com/channels/1186424376300679209/1395018119050952825) ​

### Answer The response event schema for the Custom LLM WebSocket only documents these fields:

response_type

,

response_id

,

content

,

content_complete

,

no_interruption_allowed

,

end_call

,

transfer_number

,

show_transferee_as_caller

, and

digit_to_press

[(1) ]() . There is no

cold_transfer_mode

field, and no option to select SIP REFER vs SIP INVITE, documented for this event. The choice between SIP INVITE and SIP REFER as cold transfer modes is documented only for the call transfer node in Conversation Flow [(2) ]() , which is a separate framework from custom LLM integrations. That option is not exposed in the LLM WebSocket protocol [(1) ]() . This lines up with what's discussed in a Discord thread, where a Retell team member confirmed that custom LLM transfers are limited to setting

transfer_number

in the response event for a cold transfer, and that features like caller ID control aren't available through that schema [(3) ]() . So there's currently no documented way to trigger SIP REFER from the Custom LLM WebSocket response event — sending

cold_transfer_mode

alongside

transfer_number

isn't a supported field, which is consistent with it being ignored.

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.

@AI Evy is there any previous conversation about this in this discord server?

### Answer Yes, there are two related discussions in this server: - A thread about call transfer with a custom LLM and a custom SIP trunk (without Twilio) , where a user tried transferring calls to their own SIP proxy instead of Twilio, and found that the transfer worked when the Termination URI was set to a Twilio SIP trunk, but not when set to their own SIP proxy hostname/IP [(1) ]() . - The "Phone transfer with custom LLM" thread (the one already referenced in this conversation) , where a Retell team member confirmed custom LLM transfers are limited to setting

transfer_number

in the response event for a cold transfer, that caller ID control isn't available in that schema, and that features like three-way bridging, hold music, and human detection remain unavailable for custom LLM warm transfer [(2) ]() . That same thread also links to another related discussion in the server . Neither thread mentions a

cold_transfer_mode

field or a way to select SIP REFER vs SIP INVITE from the Custom LLM WebSocket response event.

​ To learn more, see the sources I considered: [[1] Call transfer on custom LLM and custom Sip trunk (without Twilio)](https://discord.com/channels/1186424376300679209/1308509166762987520) [[2] Phone transfer with custom LLM](https://discord.com/channels/1186424376300679209/1395018119050952825) ​

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.