Definition
A cold transfer is a call handoff that sends the caller straight to another number or queue without the first party speaking to whoever answers. It is also called a blind or unannounced transfer.
Read the full guide: What is a warm transfer? Warm vs cold transfer
This is the short definition. The warm transfer guide compares warm and cold transfers in full.
How a cold transfer works
The first party asks the network to move the call, and does not wait to see who picks up. In SIP the request is a REFER message, defined in RFC 3515. It names a new destination, the caller’s side places a call there, and the first party then drops out. The first party can follow the attempt through NOTIFY messages, which also report a failure. The caller hears ringing and then a new voice, so anything the caller already said has to be repeated unless the system passes it on. Compare the warm transfer, where the first party stays on the line until the target agrees.
Why it matters for voice agents
A cold transfer is the simplest handoff for an agent to perform. It needs a destination number or SIP address and a rule for when to use it, with no second call to manage. The cost is context: the person who answers knows only what their screen shows.
Cartesia Managed Agents transfer calls with SIP REFER. On a SIP trunk, if the carrier rejects a transfer, Cartesia keeps the original call connected, so the caller is not dropped. The guide explains how to pass a summary along with a cold transfer.