Blind Transfer

In a blind transfer, you are on a call, you choose transfer, enter a number, and the call leaves your phone at once. You do not know whether the destination answers. The caller hears ringing and then either the new party or voicemail. The alternative name, unattended transfer, describes the same thing.

SIP implements this with the REFER method defined in RFC 3515. Your softphone sends a REFER to the other party, or more commonly to the PBX in the middle, with a Refer-To header naming the new destination. The recipient then sends a new INVITE to that destination and reports progress back with NOTIFY messages. When the new call connects, the original call leg to you is ended with a BYE. RFC 5589 describes the complete transfer flows, including what happens when the destination is busy or does not answer.

Blind transfer is fast and suits situations where you know the destination is the right one, such as sending a caller to a colleague's voicemail or to a department line. When you want to announce the caller or check that someone is available first, use an attended transfer.

SIPdia supports blind transfer from the in-call screen on both macOS and iOS.