Attended Transfer

An attended transfer gives you a chance to speak to the destination before handing over the caller. The sequence is: put the current call on hold, place a second call to the destination, explain who is waiting, then complete the transfer. The caller and the destination are connected and you drop out. If the destination does not answer or declines, you can return to the original call instead.

In SIP terms, you now have two dialogs. To join them, your softphone sends a REFER to one party with a Refer-To header that includes a Replaces parameter, defined in RFC 3891, identifying the other dialog. The recipient sends an INVITE with a Replaces header to that dialog, the PBX or far end swaps the call legs, and your two original calls are ended. RFC 5589 walks through the full exchange and the variations that different endpoints use.

Attended transfer is the polite option for receptionists and anyone handling callers who should not be surprised by an unannounced hand-off. It depends on the PBX handling Replaces correctly, which most Asterisk-based systems do.

SIPdia supports attended transfer between two open calls on both macOS and iOS: place the first call on hold, make or take the second call, then choose transfer.