What is the difference between transfer and redirect in SIP

Question

Is there any difference between the two call scenarios involving a SIP session? 1) Call is transfered. 2) Call is re-directed.

Answer 1

In a transfer a SIP User Agent has actually established a dialog with the callee, and then initiates setting up a new dialog between the callee and another UA.

A redirect is when a UA doesn't answer the call, but simply informs the callee to resend the INVITE to another SIP URI.

In particular, a transfer will involve REFER and NOTIFY messages, meaning that the UAs involved must support the refer event package. This package is defined outside the core SIP RFCs, where redirection is covered by the main/central RFC.