Blind transfers

The caller calls the application, the application connects the caller to the called party then disconnects from the call. There is no feedback on the success or failure of the operation.

The SIP audio provider performs blind transfers, using the REFER message followed by a BYE, per RFC 2543. However, certain gateways use the Also header in the BYE method to support blind transfers. The disadvantage of using the BYE(Also) method is that if a gateway used does not support the Also header, the caller will be disconnected.

In a blind transfer, the caller (party A) calls the application and requests a transfer to the called party (party C). The application responds by sending a REFER message back to party A with party C’s contact information. If party A responds with either a 200 OK or 202 ACCEPTED message, the transfer is considered complete and the call leg is dropped. (A call leg is connection between two addressable endpoints.) If any other response is received from A, indicating that the REFER failed, the audio provider resends party C’s contact information using the BYE(Also) method.

When there is AAI (Application-to-Application Information) in a Blind transfer, it is handled as specified by the audio.sip.AaiHeaderOnBlind value. For more information on AAI, see Application to application information (AAI).