Hello,

I’ve been searching the internet to find an explanation on how SIP transfer works using Re-INVITE and/or UPDATE, but I can’t seem to find a good source.

From what I understand(and this is the way we do it), the following happens:

Bob=Caller

Alice=Called

John=Transfer party

1) Bob calls Alice. The usual INVITE,Trying,200 OK, ACK.

2) Alice transfers the call to John using Re-INVITE.

a. Alice calls John. The usual INVITE,Trying,200 OK, ACK.

b. Alice Re-INVITEs Bob using INVITE with adjusted SDP.

3) Bob is connected to John through Alice in some magical way. I’m guessing because the SDP has been changed and for some reason the RTP stream flows between Bob and John through Alice?

Is this correct? If not, perhaps someone could explain it to me from scratch.

Maybe useful to know that we are using Cisco equipment for call handling (VXML and TCL scripts).

Thanks,

Grant