Features/Call Transfer/SIP Flow

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Attended Transfer

SIP Call Flow

A SIP Proxy (SER) B SIP Proxy (SER) C
#1 A initiates call to B --- INVITE SDP ---> | --- INVITE SDP --->
<--- 100 trying --- |
#2 B is ringing <--- 180 Ringing --- | <--- 180 Ringing ---
#3 B accepts the call <--- 200 SDP OK --- | <--- 200 SDP OK ---
#4 A acknowledges --- ACK ---> | --- ACK --->
#5 A and B talk <--- RTP (Audio) --->
#6 B places A on Hold <--- INVITE SDP --- | <--- INVITE SDP ---
#7 A accepts Hold --- 200 SDP OK ---> | --- 200 SDP OK --->
#8 B acknowledges <--- ACK --- | <--- ACK ---
#9 B initiates call to C --- INVITE SDP ---> | --- INVITE SDP --->
#10 C is ringing <--- 100 Trying --- |
#11 C is ringing <--- 180 Ringing --- | <--- 180 Ringing ---
#12 C accepts the call <--- 200 SDP OK --- | <--- 200 SDP OK ---
#13 B acknowledges --- ACK ---> | --- ACK --->
#14 B announces transfer to A <--- RTP (Audio) --->
#15 B presses "transfer" and C is automatically placed on hold --- INVITE SDP ---> | --- INVITE SDP --->
#16 placing C on hold <--- 100 Trying --- |
#17 C accepts hold <--- 200 SDP OK --- | <--- 200 SDP OK ---
#18 B acknowledges --- ACK ---> | --- ACK --->
#19 B presses "transfer" again and refers A to C <--- REFER --- | <--- REFER ---
#20 A accepts the next call goes to C --- 202 Accepted ---> | --- 202 Accepted --->
#21 A initiates call to C --- INVITE SDP ---> | --- INVITE SDP ----------------------------------------------------------------------------->
#22 C accepts the call from A <--- 200 SDP OK --- | <--- 200 SDP OK ------------------------------------------------------------------------------
#23 C terminates B call <--- BYE --- | <--- BYE ---
#24 B closes C call --- OK ---> | --- OK --->
#25 A informs B to terminate call --- NOTIFY ---> | --- NOTIFY --->
#26 A acknowledges --- ACK ---> | --- ACK ------------------------------------------------------------------------------------------------->
#27 A talks to C <---------------------------------------------------- RTP (Audio) ---------------------------------------------------->
#28 B receives NOTIFY <--- OK --- | <--- OK ---
#29 B terminates A call <--- BYE --- | <--- BYE ---
#30 A closes B call --- OK ---> | --- OK --->
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