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<@1145576664366055464> what SIP method is used in warm transfer? Is it SIP Refer or something else? Can there be an additional lag when talking to callers (after transfer) due to this method?

Thanks for tagging me <@803292707833118730>. I'll look through relevant sources and get back to you shortly.

### Answer
 Warm transfers in Retell AI use **SIP DIAL**, not SIP REFER [(1) ](<https://docs.retellai.com/build/single-multi-prompt/transfer-call>) [(2) ](<https://docs.retellai.com/build/conversation-flow/call-transfer-node>) . When using warm transfer, the system sets the `from` and `P-Asserted-Identity` headers to the user's number .

In contrast, cold transfers can use either **SIP INVITE** (the default) or **SIP REFER** [(2) ](<https://docs.retellai.com/build/conversation-flow/call-transfer-node>) . SIP INVITE establishes or updates the active call path and then bridges the transfer, while SIP REFER asks an endpoint to start a separate call to a third party for transfer handoff .

Regarding lag after transfer, the sources don't specifically document additional latency introduced by the SIP DIAL method used in warm transfers. However, warm transfers include additional steps compared to cold transfers - such as human detection, optional whisper messages, and three-way introductions - which occur *before* connecting the original caller to the transfer target [(1) ](<https://docs.retellai.com/build/single-multi-prompt/transfer-call>) [(2) ](<https://docs.retellai.com/build/conversation-flow/call-transfer-node>) [(3) ](<https://www.retellai.com/blog/effortless-handoffs-with-retell-ais-warm-transfer-feature>) . These features ensure a smooth handoff but happen during the transfer process, not after the caller is connected to the human agent.

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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)
[[3] Warm Transfer vs Cold Transfer - The AI Advantage in Call Centers | Retell AI](https://www.retellai.com/blog/effortless-handoffs-with-retell-ais-warm-transfer-feature)
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Feel free to tag <@1145576664366055464> with additional questions. I also appreciate feedback, it helps me improve.