Definition
A warm transfer is a call handoff in which the first party speaks to the receiving person, explains the caller's situation, and only then connects the caller. It is also called an attended or announced 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 warm transfer works
The caller is placed on hold. The first agent opens a second call to the target, explains who is calling and why, and once the target accepts, joins or hands over the caller. On a SIP network this is an attended transfer. RFC 5589 describes it as the transferring party first calling the target, then sending a REFER message with a Replaces header. That tells the caller’s side to call the target and take over the consultation call. The consultation is what costs time: the caller waits on hold, and the target may be busy or not answer, in which case the first agent has to return to the caller and try another route. The cold transfer skips the consultation call.
Why it matters for voice agents
Callers repeat themselves when the next person starts blind, and a warm transfer prevents that at the cost of hold time and a second leg of the call. A voice agent can get most of the benefit without the consultation. It can tell the caller where they are going, and write a summary into the record the human will open before moving the call.
Cartesia Managed Agents transfer calls with SIP REFER as a cold transfer, so a summary delivered by a webhook tool stands in for the introduction. The guide covers when to choose each type and gives a script.