### Answer Yes, warm transfer is available when using both Retell LLM and Custom LLM [(1) ]() . The warm transfer feature works with Retell's telephony infrastructure regardless of which LLM you're using to power your agent. You can configure warm transfer in two ways: 1. Using the Transfer Call Tool (for Single/Multi-Prompt agents) 2. Using the Call Transfer Node (for Conversation Flow agents) [(2) ]() Both methods support warm transfer with features like [(1) ]() : - On-hold music for the caller - Navigate IVR capabilities - Human detection to verify a human answered - Auto-greet functionality - Agent detection timeout - Whisper message (spoken privately to the transfer target) - Three-way message (heard by both caller and transfer target) The warm transfer functionality is part of Retell's telephony layer, so it works independently of whether you're using Retell LLM or a custom LLM implementation .

​ To learn more, see the sources I considered: [[1] Transfer call](https://docs.retellai.com/build/single-multi-prompt/transfer-call) [[2] Call Transfer Node](https://docs.retellai.com/build/conversation-flow/call-transfer-node) ​

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

### Answer No, the approach you've outlined won't work. The Response Event schema for Custom LLM integrations doesn't support warm transfer configuration [(1) ]() . According to the LLM WebSocket documentation, the Response Event only supports the

transfer_number

field for cold transfers . There's no

transfer_option

or similar parameter to configure warm transfer settings like whisper messages, three-way messages, human detection, or on-hold music . Discord community discussions confirm this limitation. When users asked about warm transfer with Custom LLM, Retell support indicated that "actual three-way bridging, hold music, and human detection remain unavailable" for Custom LLM integrations [(2) ]() . Users were advised they could only "simulate it client-side by sending a pre-transfer whisper from your custom LLM before triggering the transfer, then setting 'transfer_number' in the response event to perform a cold transfer" . Your options: 1. Use Retell LLM instead - Warm transfer with full features (whisper messages, three-way messages, human detection, on-hold music) is only available when using Retell's built-in LLM with the Transfer Call Tool [(3) ]() [(4) ]() 2. Implement your own transfer logic - Some users have worked around this by using the SIP endpoint integration where they dial themselves using Twilio and implement their own transfer logic with full caller ID control [(5) ]() The warm transfer features you're looking for are currently exclusive to Retell LLM agents and aren't exposed through the Custom LLM WebSo

cket API [(1) ]() [(2) ]() .

​ To learn more, see the sources I considered: [[1] LLM WebSocket](https://docs.retellai.com/api-references/llm-websocket) [[2] Phone transfer with custom LLM](https://discord.com/channels/1186424376300679209/1395018119050952825) [[3] Transfer call](https://docs.retellai.com/build/single-multi-prompt/transfer-call) [[4] Call Transfer Node](https://docs.retellai.com/build/conversation-flow/call-transfer-node) [[5] Call transfer transferee caller id for custom LLM](https://discord.com/channels/1186424376300679209/1298315652351791125) ​

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